It looks like the IPC356 format lexer/parser isn't expecting a unit value of `CUST`, but will correctly handle a value of `CUST 0`.
Currently, if I use a value of `CUST`, it looks like the tool falls back to a value of `SI`. I believe that `CUST` is permitted in the standard as an alias for `CUST 0`. See page 2 of this pdf: https://web.pa.msu.edu/hep/atlas/l1calo ... t_list.pdf