"Subject expertise" refers to a person's deep knowledge and experience in a specific area or field. It means they have a good understanding and skill in that particular subject. Full definition
Because active learning requires practice and feedback on thinking like an expert (a scientist), it demands considerably greater subject expertise by the teacher. (scientificamerican.com)
Some of them still think middle schools are junior versions of high school, and that they are justified in focusing only on subject expertise rather than student expertise. (amle.org)
Or you might seek guidance within your practice area for subject expertise but seek out someone on the management committee for advice on general firm practices. (slaw.ca)