Invoice Tolerance vs. Services Tolerance in Supplier Sites

In Suppliers setup under Oracles Payables for Oracle EBS R12.1 Invoice Management section has setup for Supplier Sites at the bottom of the page.  Users can create a supplier sites where the user can select an Invoice Tolerance from the list of tolerances created in the Invoice Tolerances Template (Payables Manager/Setup/Invoice/Tolerances) and the user can select a Services Tolerance.

What is the difference between Invoice Tolerance and Services Tolerance?  We have setup several Invoice Tolerance templates (both type of Goods and Services) and those templates can be selected for the Invoice Tolerance field in the Supplier Site setup under Invoice Management of Suppliers.  However, when trying to select a Services Tolerance there are no options to choose from.  This makes me believe Invoice Tolerance and Services Tolerance are different.  Not sure then where to define the templates for Services Tolerance or what exactly a Services Tolerance does.