A good example I have of using the post-Xlate UPoC is message modification. Most things can be done in an Xlate, but some things are just faster and easier to do in a TCL script.
For my example, our system generates an ORU results with a single OBX segment where the entire body of text resides in OBX:5 (a repeating field). There are several recipients of this message, but one of them (McKesson) wanted the results in multiple OBX segments where OBX:5 was no more than 80 characters long. Yes, this was a long time ago…
The script I wrote for that scenario broke up the body text into lines of 80 or less, but it also accounted for word wrapping and paragraph breaks. Theis is not something easily done in an Xlate, even today.
By using this script post-Xlate, I did not have to change the general ORU Xlate variant we were already using for several other systems. Had I made it pre-Xlate, I would have had to changed the variant and the Xlate to iterate through the numerous OBXs created by the script.
There are many more examples where having the script post-Xlate and not pre-Xlate or in the OB TPS made my day easier, but I hope you get the picture from this small one.