A junior scrum master should understand and be able to explain the fundamental principles and founding ideas behind agile. They should be able to explain to the more reluctant members of a team why they are doing Agile.
JSM's (if I can shorten it that way) should be able to facilitate and mediate all of the basic Agile ceremonies at a very informal, casual level. This would include daily standups, sprint planning, backlog refinement, sprint review, and retrospective meetings. They should understand the basic purpose of each meeting and provide the team some level of direction, as well as steer the meeting without meddling. Allow the right conversations to happen naturally, but also steer them away from going off on a tangent.
At this level, the scrum master needs to get better at understanding the team's work, and explain it or provide updates on things if required. ISM's should start helping with work intake. They should be able to have basic discussions with the team's business analyst and other stakeholders to help define and break down requirements and document work, and to make sure the requirements and the work intake capacity are in agreement with team goals and previous sprint capacities. They should be able to work with the product owner to understand priorities and should then ensure the team's sprint plans agree with those priorities, and help the team communicate to the PO any delays, roadblocks, or unexpected work coming in.
The scrum master should also act as a roadblock-mover. They should be able to help get things escalated and resolved so that the team can work more efficiently, and they should start to look at inter-team dynamics. For example, during retrospectives, they should begin to see if certain team members are causing trouble for others, and look at ways to remedy that. They should feel more comfortable having difficult discussions when necessary ("your behavior is hurting your team, what ideas do you have to fix that?"). Not taking on the role of manager, but they do have to manage certain things day to day and make sure the team is meeting their commitments. The scrum master should feel like an integral part of the team and should celebrate their team's successes and feel badly about things that slow their team down.
ISM's should help the team manage their backlog in terms of the grunt work of making cards and filling requirement details in and prioritizing work from discussions with stakeholders; they should be able to take a few notes as to what needs done and go do it later, without taking valuable time during the Agile meetings that could be used for team collaboration and discussion.
Meetings should become more structured and formalized. The team backlog should be well documented, either digitally or physically, and some effort should be made to ensure that cards are moved at each standup, and team members should be held accountable for providing an update. If they are not able to be physically present at the meeting, they should provide an update through email or some other means, having someone else fill in for them. The team should feel like they are in sync. There should be a working agreements document posted somewhere and the entire team should be aware of it and have voted it into practice.
ISM's should begin helping their team to become more self-organizing and to rely on the scrum master less and less. The greater their skill, the less they should be required, sort of shrinking into the background and being less of an active participant and more of an enabler and an observer.
At the senior level, scrum masters should build up a refined sense of their team's weaknesses and strengths and should develop a plan to specifically address those findings. They should feel comfortable addressing the team as a whole and with a certain amount of authority, only to motivate and enable them to do their best work. They should be able to address infractions and encourage the team to further adopt Agile practices. At this level the shift from "doing Agile" to "being Agile" should be complete. They should understand when their team feels uncomfortable with or resistant to being Agile, even when those concerns are not voiced out loud.
They should be able to get ahead of the game and help the team react to disruptors, getting resolution quickly or at least helping the team communicate the roadblocks effectively and work out what to do with the appropriate stakeholders. They should be experts at getting feedback and improving, not just the team but also themselves and how they facilitate the Agile ceremonies. They should use each meeting well, digging into the depths of the team's workload, publishing each triumph, and when issues occur, getting to the crux of what went wrong, and how to fix it going forward.
They should hold their team accountable to their working agreements and not be afraid to challenge team members and get tough when required. This should still be done in an Agile frame of mind, being willing to adapt if the team decides as a whole that things should change. Getting a feel for where the team is at and what they are thinking should be easy at this point. The working agreement document should be further refined and capture team policies for Agile meetings and learnings from past mistakes. The senior scrum master should be able to mediate and resolve conflicts between team members and to keep team meetings focused and productive.
At this stage they should ensure that all work is broken down into sprint size tasks prior to the beginning of a sprint, and that during the sprint the work is broken down further into day-size work or smaller, so that cards can be moved during the daily standup. Standups should be mandatory at this point, though the schedule would be somewhat flexible.
The retrospective in particular should be quite detailed and should really get the team thinking. Senior scrum masters know how to finesse every last detail from their team without having to beg for it, and how to help each person participate and enjoy doing so.
The SSM should become the "man behind the curtain" - helping the team along in their Agile maturity without interfering and teaching the team to run the scrum processes on their own more and more.
Scrum master gurus should be able to teach what they know and mentor new scrum masters. They should know Agile inside and out and be able to explain and defend its core tenets easily. They should be able to tailor their approach to an individual team and should be experts at reading people and understanding the team's interactions.
They should have a great enthusiasm for Agile and be able to relate any situation to its corresponding Agile principle. They should be able to perform the scrum ceremonies admirably for any team, whether brand new to Agile or quite mature. They should have multiple plans and ideas for every aspect of being Agile, and be able to quickly select the best one that applies to each scenario or to each team or even individual. They should be able to quickly build a deep technical knowledge of their team's workload that enables their team to be far more productive and efficient. They should be able to explain the work the team does in detail, and make minor decisions related to work intake, and should push back on attempts to overfill the team backlog or to hold the team to a previous velocity.
They should be able to teach the team to self-organize and manage their own backlog, and handle the Agile meetings and ceremonies smoothly and without intervention or moderation. They should exemplify Agile practices and be able to take criticism from the team, even requesting it, and use it to improve themselves. They should be able to help the team improve but to do so while being basically invisible.