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Category rank is your rank in a specific or particular category.

For competitive exams like JEE,CET,GATE,NEET etc there are students who apply from general category,SC/ST,OBC,NT, other reserved minority students and for example you are a OBC student so out of 120 say 10 are OBC with a score of 100 you may be ranked 1 in your category,here OBC. So overall you will be 20th ranked person but in your category you may be 1st.

All the best.