Part of the purpose of a card is to control the match. Players shouldn't be swearing at each other.
But when you have opponents antagonising each other, this can easily escalate into retaliatory attacks - cheap shots in a tackle, off-the-ball shots, squaring up and pushing after a challenge. It can easily escalate and keep escalating. And that's because it inflames the physical contest that occurs in the game.
When teammates are swearing at each other, it's less of an issue - they're not tackling each other, so they're much less likely to boil over. It's very, very rare for it to cause a problem.
And personally, unless it's really crossing the line, I'm not going to intervene. Because if a team wants to self destruct, I think it's unfair to the opponents if I stop that.
But if you believe that the language is starting to cross the line, it's going on too long, or it's getting a bit too aggro, by all means, have a word. You can caution them. I don't believe in the 'you can't caution if they're swearing' - that makes absolutely no sense to me. You're cautioning for the aggression, the inflammatory remarks. Heck, you know what? USB is vague, you can pretty much come up with whatever you want . Sometimes I want to book a player for a particularly tragic haircut....
If it crosses the line into, say, homophobic or racial slurs, then it has to be a red. Threats, you also need to take pretty seriously.
This reminds me, I once heard a player yell out to somebody else (I thought an opponent) "I'll f*** you up you [bundle of sticks]". Immediate red.
"I was talking to my teammate!!"
"So?"
Still a red either way!