I'd like to understand the difference, I use milestones simply to apply my monthly retainer, but I am not sure that this is a best practice.
You can use milestones in different ways. A 'billable milestone' allows you to create fixed fee invoices. A regular milestone can represent a date something is due.
TimeSolv allows you to keep things very simple or to build out Matter Plans that include phases, tasks, and budgets. If you are just doing a fixed fee invoice, you will not need to worry about phases, etc. If you wish to learn more about matter plans, please visit https://help.timesolv.com/article/317-managing-tasks