PST functions are available for YT-3300 and YT-3400 series as of right now and will be available for all of YTC smart positioners.  PST <partial stroke test> is mainly used in ESD <emergency shut-down> valve.
To highlight some advantages from PST functions are..

a) Identify valve disc / seat mal-functions
b) Identify valve stem / gland packing mal-functions
c) Identify actuator mal-functions
d) Identify any other pneumatic equipment mal-functions
e) Improve SIL level
f) Eliminate by-pass line or valve
g) Extend FST <full stroke test> time intervals