With an 85 percentile in JEE Mains under the EWS category, your chances of securing a "B category" seat depend heavily on what you mean by "B category" and the specific colleges you are targeting. Generally, in the context of private engineering colleges, a "B category" seat often refers to seats allocated under management quota or with a higher fee structure compared to the general merit ("A category") seats.

For NITs, IIITs, and GFTIs: Admission is primarily based on your JEE Main rank obtained through the Joint Seat Allocation Authority (JoSAA) counseling. With an 85 percentile in the EWS category, you have a reasonable chance of getting a seat in some of the National Institutes of Technology (NITs), especially the newer ones or those in slightly less preferred locations. However, securing a seat in the top-tier NITs, particularly for popular branches like Computer Science Engineering, might be challenging at this percentile. The EWS cutoff percentile for NITs can vary each year and across different institutes and branches. It's crucial to check the previous year's cutoff ranks for the EWS category for specific NITs to get a better understanding of your possibilities. Keep in mind that the actual ranks corresponding to the 85 percentile can also vary slightly from year to year.

For Private Engineering Colleges: If "B category" refers to a specific quota or higher-fee seats in private colleges, then your JEE Main percentile will still be a significant factor for many reputable institutions. While some private colleges might have management quotas with different criteria, a good JEE Main percentile like 85 will likely improve your chances of getting a seat in the CSE or other branches, even under a "B category" if it exists.

Counseling Process: To explore your options, you need to actively participate in the JoSAA counseling for NITs, IIITs, and GFTIs. During choice filling, you can list the institutes and branches you are interested in. If you are also considering private colleges, you'll need to follow their individual admission procedures, which might involve separate counseling or application processes where your JEE Main score will be considered.

In summary, while an 85 percentile in the EWS category gives you a good starting point, whether you can get a "B category" seat depends on the specific definition of that category and the admission policies of the colleges you are aiming for. Researching the cutoff trends for the EWS category in your desired NITs and looking into the admission criteria of private colleges for their "B category" seats will provide a clearer picture. Active participation in the counseling processes is essential to secure a seat.

I hope this helps.