Entries are a till working by the second. When race numbers vanish in two hours, the software decides the year's revenue, not the marketing campaign.
The race number ties everything together: timing, shirt size, insurance and result. Break that link and there is nothing left to repair on race day.
Transferring an entry is the commonest request before any event. A self-service form for changing name or distance saves a whole desk in the office.
The supplier plan is the real engineering project. Stage, barriers, medical post and timing have delivery and rigging slots that block one another.
Costing per event has to exist before the tender goes in. Cost per participant and the break-even threshold decide whether bidding makes any sense at all.
Billing the client and billing the runners are two separate flows. A council pays against an invoice, four thousand people pay by card, and the two have to reconcile.