A surety is the third party in a bond transaction. The contractor (principal) gets bonded by the surety in favor of the owner (obligee). If the contractor fails to meet its obligations, the surety steps in to either fund completion, pay claimants, or cover the owner's losses up to the bond amount. The surety is not insurance in the traditional sense: it expects to be reimbursed by the contractor for any payments it makes.
Sureties underwrite contractors carefully because they expect zero net loss across their portfolio. Underwriting looks at: working capital, financial track record, project completion history, key personnel experience, and project type. Bonding capacity (the maximum value of bonded work the surety will support at any given time) is a critical resource for contractors pursuing public works and large commercial projects. Building bonding capacity over years through consistent performance and strong financials is a strategic priority for any contractor planning to scale into public work.