Earmarking option states the below meaning:
Reserved category candidate entitled to admission on the basis of merit in an open category seat will have the option of taking admission either against his/her respective category seat or on an open category seat. If he/she opts for the reserved (sic open) category he/she will be deemed to have been admitted as an open category candidate and not as reserved category candidate. One seat in the college where he/she was eligible for admission against open seat shall be earmarked for a candidate belonging to the respective reserved category. Such earmarked seat will be made available immediately to the successive reserved category candidate from the same category of merit during the counseling.

Hope this helps.