Hmm, I thought lbf was to distinguish the force unit from the mass unit (1 lbf = G * 1lb mass)

It seems the common thread is that the f means to introduce G, but not exactly. In my own research, the AI summaries are about as sloppy as I've ever seen, due to the vague and often regional differences (with the difference between ft-lb and lb-ft sometimes being described as relevant, as well).