Dear candidate, NIT+ system refers to all the institutes that will give admission purely on the basis of JEE MAINS. This system includes all the IITs, Nits & GFTIs. Whereas the second system is IIT system in which the admission is solely on the basis of rank in JEE ADVANCED. 

Further more , if a candidate has accepted an allotted seat in an NIT+ Institute in round first and then in any subsequent round he or she is being allotted a seat in an IIT, then such a person will have to report to any one of the reporting centers of IITs for verification of documents. Similarly, if a candidate has accepted a seat in any of the IITs in one round and gets allotted in NIT+ institute, then again he/ she will have to report at any one of the NIT+ reporting centre. Otherwise, he/ she will have both the seats forfeited.

All the best. 

Hope this helps.