Hello dear,
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Mop-up round is actually a last step for admission in NEET.
Its a counselling round provided by states against the reverted seats.
The seats which remain vacant after 2 counselings are filled by this mop-up round.
In simply,Its a chance to get a seat if seat is not allowed in 1st and 2nd Counseling
Its same like other counseling.Its a last round for admission.
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Even seat is not allotted in this round,you should try another year for admission.
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Hope i have cleared your question
Thank you