Service Contracts applied/selected in Service Requests

Hello  everyone,

I'm not sure this is the right place for this question, but if someone can help me, or indicate me the right domain ...

We try to undurstand the way the eligible  Service Contracts  (entitlements) are selected (checked) when opening a new Service Request ?

Context : we have some  installed base products which are covered simultaneously by a  service agreement and a warrenty (or by two service agreements).

When opening manually a new SR , we can see that one coverage line is automatically checked, and in that case it's no problem because the user can modifiy this selection to apply anothe one.