If a Plan/Program template, different to that set on the Service Contract, is required on a Case then this can be assigned to the Case uniquely in the Case List > Plan/Program tab using the Finance Templates link above the table:

Clicking this link will open the Finance Templates window for the Case where templates for both Plan/Program types, Monetary Based and Time Based can be assigned:

If no unique template is assigned at a Case level, then this will automatically be set to "Service Contract (default)" which will follow the rules of the financial template set on the Service Contract that the Case is assigned to.