Hi,
It acts like a hierarchy basically and you will decide based on the adaptability of your current environment or the business blueprint approach you will following.
Lets take an example:-
First of all as per the best practices, you will either create a master solution in solution manager for all of your landscape usage and you will incorporate all the details in that solution. Now for the enhancements and implementations or upgrades/Version releases (major or minor) you will utilize your master solution and documentation present in that. You might need to change the existing structure based on your implementation which you will consume in the maintenance project inherited from the solution.
The same thing you can perform via first creating a template project and then create a solution for the same.
Master solution will be used for the business process monitoring as well as it will be solution based and not project based.
While change management will be project based (as of now 7.1) however if you are following the hierarchy it can incorporate the solution also.
So it is the way you are architecture the blueprint based on your client scenario.
Thanks
Rishav