You've been looking around at safety harnesses and they've all got different buckles. Some have mating buckles, and other have pass thru buckles. What in the blue hell is the difference? We'll tell ya.
Nothing. They are the exact same thing... essentially. The basic components are the same. They're simply two strips of metal that connect together. One is male and the other is female. And when a man buckle loves a woman buckle... Um, is this getting uncomfortable? Go ask your mother.
Basically, one side of the mating buckle has an opening that allows the other side of the buckle to go through. (That didn't sound much better did it?) After the buckle is connected, it is virtually impossible to disconnect them unless you slide the male side back through perpendicular to the female buckle. So in the case of a fall, when pressure is put on the buckle, they press together back to back and you stay in your crotch restrictor.
So that's a mating buckle. The pass-thru buckles work the same way, except they have a courtesy notch at the top and bottom of the female side to make it easier to connect. It's really nothing special. In fact, most people use these phrases interchangeably. I do. So if you ever have a question about whether a particular harness has the extra notch let me know and I'll let you know.
So, as you spend time searching online and you come across these variations, you'll know what they're talking about.
P.S. This is one of the most common buckles in harness production. You've probably used this buckle before without even noticing it.