This is a good question. Most people (escorts and clients) consider LFK as kissing without using tongue. I think it's wrong. French Kiss must involve tongue. Light French Kiss should mean kissing with a little bit of tongue.
I think differentiating DFK and LFK is arbitrary. If you search up the definition of a French kiss, it says it involves tongue on tongue or tongue on lip action. So what really separates DFK from LFK is indeed just how much tongue action there is, or the duration of the overall French kissing, which can be subjective between us.