I've never been sure of this, so, what happens when I don't define a variable like GibWoundLimit?

As far as I know, you can create actors (or AEmitter or MOSRotating) without defining that variable, without defining GibImpulseLimit or LifeTime.

Does that make that variable set to infinite or something?

From my experiences, I do believe infinite is default if you don't define a variable.

Unless it's something where the number sets a status to something, like HitsMOs or DrawAfterParent then it would either give an error or automatically set it to a default number depending on what the variable is.

EDIT: Jesus Site lag.

may be slightly off-topic, but this
do works.
("Null" is a seperately defined atomgroup)

It seems it is set to infinite. Anyway, it's better not defining the variable that spamming 9s.

I like to leave the variable there, but commented out, when I want a default value. That way it's slightly more intuitive to change it from default to another value later when adjusting something, or copying the object when making something similar later.

Not all variables are infinite by default, though. Sharpness, AirResistance, GlobalAccScalar, and plenty others that don't come to mind at the moment have 0 or 1 as default.

And sometimes infinite is treated numerically as -1, for variables where negative values would make no sense. So if infinite isn't default, or you just feel like inputting it, that might be worth considering.