Fill device designed to securely store, transport, and transfer electronically both COMSEC and TRANSEC key, designed to be backward compatible with the previous generation of COMSEC common fill devices, and programmable to support modern mission systems.
Sources:
CNSSI 4009-2015
under data transfer device (DTD) (COMSEC)
from
NSA/CSS Manual Number 3-16 (COMSEC)