The CDC (Communications Daughter Card) slot is an internal expansion slot found in modern ThinkPads.

It is not meant for customer upgrades.
Different CDC slots exist for different functions and different form-factor cards, so you cannot just install any CDC adapter into a CDC slot.

The types of CDC cards you can use in your system is dependent on the BIOS. If you insert a Bluetooth IV with 56K Modem from an R52 system (for instance) and place it into an R51 system which only had the Modem Daughter Card, the system refuses to boot up and displays Invalid Daughter Card.

If anyone knows more details, please elaborate.