Sometimes when I don't change much in a post, it says at the bottom "punctuation edited" or "lightly edited"

Just out of curiousity, what are the specifications for when it does this?

I think:

"punctuation edited" means just that.

"lightly edited" means no change to alphanumerics.  Punctuation, spacing, and formatting can change.

Not sure, though.

Actually I'm pretty sure what you said for 'lightly edited' is 'punctuation tweaked', while 'lightly edited' is if you change only a letter or two (but I'm really not sure about this).
Other than those 2 there's 'edited' (for a larger edit) and 'updated' (e.g. change of poster, if you posted as the wrong character.
This message was last edited by a moderator at 12:17, Tue 17 Apr 2007.

Okay, this is what I found...

Punctuation Tweaked:  Exactly what it says.  A non-alphanumeric character was changed in some way (added/deleted/switched for a different one).

Lightly Edited:  A single alphanumeric character was changed in some way.

Updated:  Changing something which isn't actual text (name of poster, Group, Notice setting).