Why following fields are mandatory in Contract Project type

Hi Folks,

Could some one explain me the functionality of Following fields  in Project type.

I am expecting each field purpose and usage with suitable business example.

Billing Information tab

1. Billing Job Group

2. Employee

3. Job

4. Organization

5. Schedule

6. Invoice formats "Labours" and " Non Labour"

Billing Assignments tab:-

7. Name

8. Amount

9. Percentage.