There are Two Separate trains DHN-ALLP/TATA-ALLP both run combindly from Rourkela to ALLP as a single train.So now with two portions there will two nos of S1,S2,S3 etc...So to reduce confusion Tata-Allp slip portion coaches are named as ST1,ST2,ST3 etc and other portion (DHN-ALLP) will have S1,S2,S3 etc..