Basically it works like this:

A full license is used when a user logs in if their user group does not have the modules that they can view restricted. So they can see everything in ALM.

An additional defects license would be for users that only need access to the defects module, you would make it where their usergroup ONLY has access to the defects module then when they log in it should consume an additional defects license vs a full license. Here is a post that goes into more detail:

http://eyeontesting.com/questions/7378/additional-defects-licenses.html

The Business Componenents license gets consumed when a user logs into ALM if their user group has access to the components module enabled, they also would consume a full license as well, the posts below show a few scenarios on BPT licenses, also on how to limit which users have access to the licenses so they don't all get consumed:

http://eyeontesting.com/questions/14840/uft-alm-bpt-license-questions.html

http://eyeontesting.com/questions/6153/how-do-we-limit-bpt-license-usage.html