The following number register names are used by the ms package. An italicized n means that the name contains a numeral (generally the interpolated value of another number register).
nT | AJ | AV | BC | BD | BE | BH | BQ | BW | 
CW | EF | FC | FL | FM | FP | GA | GW | H1 | 
H2 | H3 | H4 | H5 | HM | HT | I0 | IF | IK | 
IM | IP | IR | IS | IT | IX | In | Jn | KG | 
KI | KM | L1 | LE | LL | LT | MC | MF | MG | 
ML | MM | MN | NA | NC | ND | NQ | NS | NX | 
OJ | PD | PE | PF | PI | PN | PO | PQ | PS | 
PX | QI | QP | RO | SJ | ST | T. | TB | TC | 
TD | TK | TN | TQ | TV | TY | TZ | VS | WF | 
XX | YE | YY | ZN | 
When you're writing your own macros, the safest bet is to use mixed-case letters for macro names. (Using uppercase letters could conflict with reserved ms names, and using lowercase letters could conflict with troff requests.)