This rule has been changed slightly. It used to read:
Do not edit or delete a post after a moderator has issued a warning or infraction to that post, without permission from a moderator.
It has been updated to this:
Do not delete, edit, or obscure either the bold red text a moderator has edited into a warned or infracted post or the offending portion of that post without permission from a moderator.
The goal with this change is really to not have to dish out penalties for harmless edits like "thanks!" or "sorry!" or just adding content to a post that has been ticketed. Modtext may not be meddled with, nor the part of the post that got you carded in the first place (since this now serves as a learning tool for other members), but other than that, you'll be good.
Additionally, if a moderator edits in < and >, that indicates what's considered the "offending part" of a post. Using this isn't common practice yet, but you might see it here and there.
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Ooooo... I'm a geek. Sorry.