I can explain this question better with an analogy.
Suppose I am a uni student and I have a maths tutorial. For this tutorial, there is attendance, a tutorial sheet and some other things. For this tutorial to be complete, I just need to attend, but the tutorial sheet is worked on during the tutorial. It is not compulsory to complete this sheet yet, but will be later for a final test.
Let's say in this "Week 1 Tutorial" I complete 4/8 of the questions on the sheet and I can no longer devote any time today to work on it. Assuming there is a manager to report to (since this is an analogy after all), they want to know what I was working on today at 12:00 - 1:00, which was the tutorial sheet. This is displayed on a software calendar via assigning a start and finish time to tasks, so in this case, I have "Math Tutorial" from 12:00 - 1:00. Since it is incomplete and I am going to work on it later, I can't re-assign the times to work on the tutorial sheet since it has already got the times of 12:00 - 1:00 set for it.
How can I understand this complication in a sort of tree-diagrammy kind of way so I can figure out what the subtasks are and dependencies are and the places that they exist in the main project of "Math 101"? This is the kind of diagram I was using to explain this, however it may not be very helpful nor the correct way to organise the issue.