Short answer: Composite rating charges the whole group the same tier rates, for example employee-only, employee+spouse, family, regardless of each person’s age, using a blended average.
Under composite rating, the insurer blends the group’s demographics into uniform tier rates that don’t change as employees age during the year. Everyone in a given tier pays the same, which simplifies payroll deductions and billing. It contrasts with age (member-level) rating, where each enrollee’s premium reflects their own age. Composite rating can benefit groups with an older workforce and makes budgeting predictable, but it can be less competitive for groups with many young employees.