A fitness or wellness business is a membership business with some retail attached. The books should make that structure obvious.
Revenue
Membership revenue — the recurring base. Class or session revenue — drop-ins and packages. Personal training or one-to-one revenue. Retail revenue — products sold on site.
Keeping memberships separate is the whole point. It is the predictable income the business is valued on and borrowed against — see recurring revenue.
Deferred revenue matters more here than in most trades
Packages and annual memberships are paid up front for service delivered later. That money is a liability until the sessions are used, not revenue — see deferred revenue.
A studio that treats prepaid packages as income looks far more profitable than it is, and the correction arrives when people turn up to use what they paid for.
Cost of delivery
Instructor and trainer pay, whether employed or contracted, and the cost of retail goods sold.
Instructor pay tied to class attendance behaves like a variable cost. Salaried instructors do not — see fixed and variable costs.
Overhead
Premises is usually the dominant line and usually the largest commitment the business has made. Equipment, booking software, insurance, marketing, cleaning.
What this setup gives you
Membership revenue as a share of the total, and the deferred balance — the two numbers that say whether the business is stable or simply busy.