Any unclaimed reimbursement allowance rolls over to the next month and accumulates to build up a balance of unclaimed reimbursements until the end of the calendar year, when it zeroes back out.
Remember, the QSEHRA is a reimbursement facility, not a savings account. The rollover amounts are allowances that can be claimed for reimbursement, rather than cash available to use anytime. At the end of a calendar year, any unclaimed allowance will be lost.
John Smith is entitled to $300 per month in tax-free health reimbursements. His company has set up the HRA to reimburse him for both monthly premiums and additional medical expenses. His monthly premium is only $150 per month, and he has not had any additional medical expenses (prescriptions, doctor visits, etc.) for three months. That means that for three months, he has accumulated $450 in unclaimed reimbursements.
In month 3, after he pays his monthly premium, John had to go to the dentist and pay for some fillings out of pocket. He can apply the $450 in accumulated unclaimed reimbursements towards his dental bill.
In the same way, a large medical expense that is more than your monthly allowance can be paid out over time. See more information about that here.