This sounds like a direct translation from Chinese. Char siu (or char siew - roast pork) can be 'lean', 'fatty' or 'half fatty and half lean'. I think 'half fatty and half lean' is used a fair bit now. In other contexts, people might talk about marbled meat - that term is more associated with beef.
If I saw "half fat, half lean meat" in a recipe, I would imagine two kinds of meat that were going to be used together (perhaps they would be minced up). Some kind of explanation, like nat's, would be necessary for someone like me who is unfamiliar with the sort of meat you are talking about.