The phrase "particular subject" refers to a specific topic or area of interest that someone is focusing on or discussing. It means narrowing down the scope to a specific thing rather than talking about everything in general. Full definition
The assignment will be 100 % original and unique as the writers hold great knowledge in particular subject area. (studentsassignmenthelp.com)
In providing professional development, teacher leaders focus on particular subject matter content or pedagogical approaches intended to build the instructional skills and abilities of classroom teachers. (mspkmd.net)
As for particular subject matter, I don't like narratives, basically. (widewalls.ch)