I am sure that there will be efforts in different organization to find resource billable vs non-billable hours in Clarity. There could be couple of options of doing this..
1. Use Resource Input Type Code
2. In Resource Financial, use Resource Type
3. Create a custom field
#1 is quick way of doing without enabling financial in Clarity. Also flexible option for the resource to change option from Timesheet
#2 needs more planning and much more efforts to implement financial
#3 is custom solution
Need to know how other organizations find billable/non-billable time, and better options