In most documents there is a section on assumptions e.g. We assume that the network is well within the realms of supporting this solution.
- At what point does it become a risk?
- Where does the risk lie e.g. project, operations, business, all?
We create assumptions because we don't have the answer to a question. If we don't have an answer to a question, that means we have uncertainty. If that uncertainty is tied to an objective, it is then a risk. This mean, the moment you create an assumption, you have a risk, and it remains a risk until either you find the assumption was correct or you mitigated the risk to the point where you can accept it.
Where it lies is dependent up on the assumption. That would be part of your risk assessment.
All assumptions should be validated at the appropriate point of the project. If they are shown to be valid assumptions (ie. correct) then that is fine. If they are invalid then they become either risks or blockers and may require change control to manage the impact on scope/cost/timeline.