Continental's terminology gets a little complicated.
Basically, on international flights that do not offer BusinessFirst service (such on 737's, some 757's, etc) CO offers Business Class in most cases. Plain Business Class is designated by 'C", "D", and "Z". These are all the same "business class" seats. If one wants to buy a business class seat from IAH to CUN, it comes out of the "C" bucket. If one wants to upgrade, it comes out of the "Z" bucket. Thus, if you see C6 D6 ZO, there are 6 seats avaialble in business class but none available, at that time, for upgrade. "D" appears to be the bucket from which the seat comes from that someone buying a businsess class ticket from London to Cancun through IAH. Whenever a "D" of "Z" ticket is issued, "C" will decrease by one.( A complication is that for domestic flights, CO will frequently have "A"(first class) and "D", in addition to "F", from which upgrades come. The theory is the same, that someone buying an international business class ticket should get a first class seat on the US domestic portion of a trip on CO).
On international flights that have BusinessFirst service, then the designations for this service are "J", "D", and "Z". What I said above applies to these.
I think I am at least 99% right on this, but welcome any comments.