Chapter 6 of the ClearCase User's Manual has a good discussion of config specs.
Some information is summarized below.
When you are ready to check something out in ClearCase, you have 3 basic choices - a reserved checkout, an unreserved checkout, and a checkout on a branch. Each type has its pros and cons. This means that you are guaranteed to be able to check the element back in without having to do a merge. It also means that others can't check the element out reserved until you put it back and that people who checkout the element unreserved, or on a branch, have to wait for you to check it back in before they can merge. Checking in MAY be a little more complicated.