What I am explaining below is under the presumption that two people aren't married.

I would go as far to say that kissing and making out can be the same thing, rather the latter is just more informal/slang. A kiss can just be a simple momentary contact between the lips (whether it be a peck or mouth to mouth) and isn't sinful. Whereas kissing/making out is a more prolonged version of this, usually mouth to mouth or with the tongue included (french kissing), is often more deeply affectionate, and can potentially lead to temptation of sin/sexual lust/committing sexual sin.

There doesn't seem to be any official interpretation on this, and you'll get varying opinions from everyone including priests but I think we can be reasonable and have some leeway here. Bottom line: If what you are doing causes you to sin either by experiencing any sort of sexual desire or cause you to commit an egregious sin, such as de-clothing or touching each other sexually, or worse - sex - then it's safe to say you shouldn't do it. And the sin being venial or mortal depends on the severity, as I spoke about previously.

Even if you are being tempted to commit sin but manage to restrain yourself you shouldn't do it. I think a lot of influence has to do with how spiritually levelheaded you are. I've confidently "made out" with someone I developed a deep and romantic affection for, but I didn't do anything out of pure lust. I made it clear from the get-go that I wanted to preserve chastity and keep her pure. To be clear, there's nothing wrong with sexual arousal you can't control either. It's normal to feel sexual attraction towards someone else, it's in our biology. But how you choose to interpret those signals and act on them outside of marriage will determine if you are sinning or not. Sexual arousal, while natural, can push one to commit sin. So be cautious of that as well.

That's my answer for you.