If you are asking about what unit
number to use with IUNVEL, that's done in a fashion analogous to IUNCRD for a coordinate trajectory, e.g.
open unit 41 write file name dyn.trj
open unit 42 write file name dyn.vel
dyna leap strt timestep 0.001 nstep 100000 -
iuncrd 41 inuvel 42 -
If you're asking about the units
for the velocities, they are Angstrom/AKMA-time, where AKMA-time is the internal time unit used in CHARMM, and 20 AKMA time units is .978 picoseconds (1 AKMA time unit = 4.888821E-14 sec).