Upsized Cashback rate - usually 5.5%.
See this deal for some of GYB's cash back tips. I assume the same T&Cs apply to this deal. 12% seems to be a somewhat frequent upsized rate (at both Cashrewards and Shopback), but is still nice if booking now. At most, it tends to go up to 12.5% - 15%.
Booking.com often increases price if you go though shopback and there is no guarantee the cashback will track.