Nier Automata (the game) has many track variations that change in real time, and variations in Japanese are labelled with 丙, 乙, 甲 and 丁. In English they correspond to (IIRC) quiet, medium, dynamic and 8-bit.
You can see some tracks with those labels in the OST and
Hacking Tracks album, so the :丁 is part of the track name.
丙/乙/甲/丁 here seem to go for an order meaning, that being 3rd/2nd/1st/4th.