Allowing separate approvers for subcategories under a single activity / milestone
Allow different approvers to be set to approve hours in subcategories under a specific milestone. Example - milestone could be hours volunteering in a home improvement program. Have a person set to approve all volunteer plumbing hours, set a different person to approve any landscape hours, set a different person to approve any electrical hours etc. All hours regardless of the approver (Plumbing, landscape, electrical etc) count toward the main home improvement category.

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Beau Abdulla commented
Example - say we have a main activity called on campus service that has a milestone of 10 volunteer hours for students to achieve. As I understand we can use conditional logic to create sub categories of on campus service - tutoring, athletics hep, TA help etc. It would be great to be able to assign approvers for each sub category and allow sub categories to have sub categories. Example with the TA's. A student first selects on campus service as the main activity. They then have drop down that Teacher's Assistant (TA) is an option. They click on that and a third conditional option (a teacher list) pops down. The student selects the teacher they TA for and inputs hours. Each teacher should be able to be added as their own hour approver.