Question on combat values and fire values

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Hi guys, I'm completely new to Squad Battles series, and to more complex wargames in general (I've only played Panzer Corps/Battle Academy/Unity of Command. I just recently learned about John Tiller games and I'm loving them. I have trouble with the manual though, some stuff I just cannot find in it, like opportunity fire rules and such. Also in the firing formula it mentions unit Strength but nowhere in the manual is defined what unit Strength actually is?

Reading this thread I understood that it's the actual number of firing units ? Is that mentioned anywhere in the documentation?

Also what is Strength of the weapon ? Is it its base fire value, like fire:3 ?

How are combat loses distributed?

One more question while I'm at it, how do undermanned (for example) machine guns work? Is it possible to make them manned again. Also can I and how pickup dropped enemy weapons? Sorry for tons of question, thanks :)