This file contains text indicating the correspondence between the key components of a CID font and CMap file names.
This file contains a list of aliases and filenames separated by a '>' for Japanese ATM fonts. It has references for User Defined Characters (UDC) of the HeiseiKakuGothic and HeiseiMincho fonts. The IBM supplied CIDFont names are IBJHKGW5.CID for Japanese Heisei Kaku Gothic (JHKG), and IBJHMNW3.CID for Japanese Heisei Mincho (JHMN).
See g5445854.pdf for more information.
It seems that ATMDATA.DAT is a leftover from using Type Transformer.