In projects with T&M on Participants, you can modify participants' rates using the modifier on task types.
It can be helpful in those cases e.g. overtime where certain resources' tasks should have other rates than the rates entered on the resource level in the Admin page.
When creating the project, set up the budget to time or fees and project data type to T&M on participants. When you add resources to the project their rates will then be taken from the Admin data.
If you want to count rates differently, for example when a resource on overtime uses 150% of their standard rate and only 90% of it during internal meetings, you can add a task type called Overtime and enter the modifier 150% and create a task type called Internal meetings with a modifier of 90%.
After entering those tasks with rate modifiers the actuals in time or in fees will be counted from the timesheet entries taking into consideration the rate modifiers.
So if you put 4 h per day for 5 days against an activity with a modifier 150% with budget in time - 30 hours will be counted for this particular resource, not 20 hours as it was entered in the timesheet. The same thing would happen if the budget was in fees.