i suppose you would go to HK downtown if u have 12 hours (i.e. will pass immigration)
because the baggage, as in all airports, in after immigration.

if you check your baggage to HKG only. you can pick them up. and check in again at HKG, before heding off to downtown.
check in desks is one stair up after you exit customs, both elevator and escalators are available to take you upstair.
CX coutners for J are on the right hand side (aisle B) after you exit customs