question about mod sec parameters

Hello

I tried to look for parameter descriptions in google but I can`t find
anythin relevant.
I have following rule

SecRule REQUEST_HEADERS:User-Agent "@pmFromFile badbots.txt"
"id:350001,rev:1,severity:2,log,msg:'BAD BOT - Detected and Blocked. '"

and my questions are :

what is ID ?
what is rev ?
what is severity ?

Cheers
Greg