Exclamation mark - what is it?

I read this line in the manual (System::Call):

OPTIONS is a list of options which control the way System plug-in behaves. Each option can be turned off by prefixing with an exclamation mark. For example: ?!e.

couldn't find any more explanation about the exclamation mark. in the bold text, e might be one of those options, I guess, but what about the "?" and "!", is there any other(s)? could anyone explain with some examples?

BTW, is nsis script case sensitive?

thx!

? >turns on the processing of "Options"
e >Call GetLastError() after procedure end and push result on stack (search for "Available Options" in the readme for a list of options)

so;
?e >will push GetLastError to the stack
?!e >will NOT push GetLastError to the stack (it could have been turned on if you used a "system macro", but each option is off by default AFAIK)

thanks, Anders, you've just lit the candle.