LAPDOS is a DOS-based PC TPDD client. It comes with a program that, loaded onto the Model T, turns the Model T laptop into a TPDD server. This is the only known instance where a Model T is made to be a TPDD server. Typically, it is the client.
This allows LAPDOS to copy files directly to/from a Model 100 without an intervening step of moving the files to a disk drive. This innovation, and a dual-pane file manager is likely what made LAPDOS such a successful product.