All trains going from UDL to SNT stops at UDL (and vice-versa). I don't know the actual reason, its just done for several years and u can find it in attached WTT Oct 2021.
Operating dept. can provide operational stop for several reasons like
•to facilitate staffs (some locals stop at CRAE,DCOP,POL),
•for...
more... passengers on demand (eg. 13045/12342 at LLH)
•crew change (eg. 13106 at Andal for driver change)
•to wait for clearance of block section (eg. 13418,15460 halts at SNT from 10:35 to 11:00 to allow 03580 start first from SNT & wait for line clearance)
•precendance (eg. 13159 at BDH to allow 12343, 13164 at BSAE to allow 13064),
and many more reasons
In case of the last 2 points mentioned above, operational stop may not be maintained if any of the train is out of path/not running, so we don't update overtake halts here.