Because these internal names are used for linking pieces of TDF together, they are collectively called linkable entities . The interface information relates these linkable entities to each other and to the world outside the capsule. The most important part of a capsule, the part which contains the real information, consists of a sequence of groups of units. Each group contains units of the same kind, and all the units of the same kind are in the same group. The groups always occur in the same order, though it is not necessary for each kind to be present.
Every capsule has exactly one tld unit.
It gives information to the TDF linker about those items in the capsule which are visible externally. These units contain information about the versions of TDF used. Every capsule will have at least one such unit. These units contain declarations for tokens. They bear the same relationship to the following tokdef units that C declarations do to C definitions.