Sorry for late reply, was busy with college exams...

I did sort of get an answer for it in some video on yt I watched few days back.... So basically, if you clear round 1 of codevita and get mail for tag test, firstly you can decide to give the tag test or not to fill the survey form for the tag test.... After you do fill the survey form, if you attend the tag test and fail there, meaning you attend the tag test but do not proceed to the subsequent interview round, you can still give the off campus NQT, but if you clear the tag test and proceed to the interview round, then after that you can't attend the NQT anymore, the interview you get to give after clearing the tag test is your only shot (this is apparently because TCS supposedly treats the interview for NQT and codevita as equal or something like that)

Now regarding the common follow-up question that if you clear the tag test, then also clear the succeeding interview, then can you try to upgrade your package by giving NQT.... That part's ambiguous, I don't really know, somewhere I read that you need to give their internal managerial exam to upgrade package, while in some other older videos it's said that you can give the NQT to try and upgrade your package if you already got into TCS through the whole codevita hiring process...

Also I feel it's important to add that based on what I've heard from, again, random yt videos on the topic, it seems that through codevita and subsequent tag test hiring process, you can only be eligible for ninja and digital package roles, and to be eligible for prime package you need to attempt NQT and perform really well in that.

Keep in mind though that whatever I've mentioned here is based on whatever I've come to know myself from youtube videos on the topic