Since I only play slots (not video poker), Borgata has two numbers to keep track of comp dollars and slot dollars, cd's are used for restaurants and other non gambling activities (merchandise is at a 2:1 rate at the shops)--cd's do NOT cover tips; slot dollars are used for "free" slot play in the machines, for the slots I play they are earned at 1/2 the rate of cd's. Right now (October and extended into November and hopefully into December) you earn sd's at 5X rate. CD's expire if unused after 6 months.
Since Borgata is linked with Mgm and Boyd casinos in LV, you can redeem Borgarta certificates (minimum $50) for use there--these cover anything you can charge to your room (including tips) and merchandise is at 1:1 ratio. Excess value on the certificates is forfeited (so I you use a $50 and charge $40) you are not supposed to get the $10 back--Bellagio has been nice and in some instances credited my cc with (even) small excesses.
PS: The lowest level is Red Card and Black Card (requiring 1000 cd's in a year) is next. There is a Titanium level, but info on this tier is not published.