This is a description from http://keison.co.uk/digium/digium_wildcard_tdm_400p.htm.  Many more details available on their website.  

The Wildcard TDM400P is a half-length PCI 2.2-compliant card that supports FXS and FXO station interfaces for connecting analog telephones and analog POTS lines through a PC. Using Digium's Asterisk Open Soruce PBX software and standard PC hardware, one can create a SOHO (Small Office Home Office) telephony environment that includes all the sophisticated features of a high-end business telephone system.

The TDM400P takes the place of an expensive channel bank and brings the system price point to a low level. By using FXO and FXS modules with the TDM400P, one can create a solution with support for a range of telephones. To scale this solution, simply add additional TDM400P cards populated with modules.

The naming convention for the TDM bundles is as follows: TDM X Y B. Where "TDM" denotes that the card is TDM, "X" denotes the number of FXS modules, "Y" denotes the number of FXO modules, and "B" indicates that that this product is a bundle.