The Optional Service Contract Terms are the standard contract terms Upwork publishes for clients and freelancers to use between themselves, applied to a contract by default unless the two parties agree on terms of their own.
Upwork is not a party to the contract between a client and a freelancer or agency; it provides the marketplace, the escrow and the payment rails under its own Terms of Service, and leaves the two parties to agree what the work is and on what terms. For the many contracts where nobody drafts anything, the Optional Service Contract Terms fill the gap. They say that the freelancer is an independent contractor, that the work is done to the client's specification, that ownership of the work product passes to the client when it is paid for, that each side keeps the other's confidential information confidential, and that neither side warrants more than it says. The parties can replace them with their own agreement, a master services agreement, a statement of work or an NDA, and where their agreement and the default terms differ, their agreement wins.
The word "optional" is the one to notice. The terms govern the relationship between the parties, not their relationship with Upwork, and Upwork does not enforce them: a dispute about whether the work met the specification goes through Upwork's dispute and mediation process only as far as the money in escrow or under payment protection is concerned, and beyond that it is a contract claim in whatever court the parties' law provides. For an agency, the agency account is the contracting party and its members are its workers under the terms.
How Upwork Insights uses it
This term describes Upwork's own rules or product rather than a figure the index measures; the pages that do carry live figures are the stats and rate pages.