It's very clear really. Her members' site at $24.95 for the first 30 days, $19.95* per 30 days after that is "extreme" non-nude - basically non-nude with some sheer outfits, the odd slips in webcams etc. She has some fixed price videos which are still non- nude but still hot.
With most non-nude models, that would be that but Briana offers the additional option of the cams on Nude Adult Cams (nac) - $8.50 private, $12.99 VIP - where you can see her completely nude. She is not the most expensive on NAC and it must work because NAC is hardly alone in the marketplace.
Briana is such a hot girl I guess her non-nude site must be doing (and will continue to do) well. While she gets a good number of people paying the NAC prices, why would she go fully nude on her members' site?
I can't think of another model who has effectively started off with this business model. I wonder if others will follow?
* - Apologies if I have these wrong - I'm in the UK and all I can see are the Sterling equivalents.