This part of the manual might help
*iso* or *aniso* or *tri* values = Pstart Pstop Pdamp
Pstart,Pstop = scalar external pressure at start/end of run (pressure units)
Pdamp = pressure damping parameter (time units)
*x* or *y* or *z* or *xy* or *yz* or *xz* values = Pstart Pstop Pdamp
Pstart,Pstop = external stress tensor component at start/end of run (pressure units)
Pdamp = stress damping parameter (time units)
Arun