Short answer: The insurer starts from expected claims and adds a corridor, a percentage set in the quote, then usually turns that into monthly aggregate factors multiplied by actual monthly enrollment. The employer pays eligible claims up to that total before aggregate reimbursement begins.
Aggregate stop-loss reimburses the employer when total eligible plan claims for the contract period exceed expected claims by more than an agreed percentage. That threshold is the aggregate attachment point, also called the corridor. The percentage is a pricing term the carrier sets, not a legal standard.
In practice, carriers convert the threshold into monthly aggregate factors (per employee or by tier). Each month’s factor times that month’s enrollment builds the annual attachment point, so it moves with enrollment.
Points to check on the quote: whether claims reimbursed under specific stop-loss are removed from the aggregate count, how much of each person’s claims (and any lasered amounts) counts toward the aggregate, whether pharmacy is included, the aggregate maximum, and whether reimbursement waits until year end or is advanced monthly. Fixed costs such as admin fees and stop-loss premium sit outside the attachment point.