Hello,

See, the IIIT take admission on the basis of JOSAA counselling. JoSAA gives you the facility to get your documents verified at any of the centers as per your convenience, these centers are otherwise called Reporting Centre or RC. These centers verify your document and ensure your eligibility for admission. After the reporting and verification, if you have frozen your choice and want to go for admission, Reporting Centres give you a seat acceptance cum verification letter. This is the very document which is mentioned as RC Letter in some of the college Websites.

I hope I cleared your doubt. 

PS: the letter downloaded from JoSAA site is the latest seat allotment letter and is not same as seat acceptance letter.