resource capacity as I understand it is used to determine how much capacity for workload it has per resource allocation. the overall capacity of workload in total should usually be 100% of the available resources working hours, spread out accross allocation.
Thus, I would describe an example that you have
Resource Pool A allocation 30%
Resource Pool B allocation 60%
Resource Pool C allocation 10%
and Staffing profile position allocation is determined by that.
That however doesn't touch the timesheet policy per se, as the user is able to add timesheet lines for any allocation according to the project settings for Time Management.
That having been said, you could use an enforcement of 'Period in Percent', in which you determine either a fixed number of hours per period or a number of workday x hours in a day per period .
Again though, I don't think that matches your desire, or I'm misunderstanding your intent.