Due to some recent issues, we've had to formalize our stance on sig lines.
We've tried to keep it short and simple:
Free expression: ok
Non-advertisement links: ok
Shaving Advertistemnt (Tasteful): ok
Other advertisement links: not ok
And for the rest the default site rules apply, so no spam, no abusive language, etc etc.
We don't want to make it too specific. If in doubt, then just ask a mod.