An employer-funded reimbursement account that pays for employees' qualifying medical expenses, with no employee contribution and no FICA-wage-base reduction.
HRAs reduce employer benefit-cost outflows but do not reduce payroll tax. A SIMERP reduces payroll tax (FICA on both sides of the line) and provides reimbursement-based benefits. They are not substitutes - many employers run both.
Definitions only do so much. A live walkthrough of how all of this connects in your business takes about half an hour.