An exclusive agency member is a freelancer who works on Upwork only through one agency, so that every contract they take is an agency contract and every payment goes to the agency's financial account rather than their own.
Upwork lets each agency member be marked exclusive or non-exclusive, and the choice decides where the money goes. For an exclusive member, Upwork's Help Center states that all earnings are paid to the agency financial account; the member cannot accept an independent contract, their profile is shown to clients with the agency's name attached, and a client who finds them in search is in effect finding the agency. The owner sets the status, and the member accepts it.
Agencies prefer exclusivity for the people they have trained, placed and are paying a salary to, because it stops a client who met a developer through the agency from re-hiring that developer directly for less. From the freelancer's side it is a trade: a steady flow of work found by someone else, against giving up their own pipeline and their own earnings record on the platform. Many exclusive members are employees of the agency in the ordinary sense, working from its office, and have never bid for a job themselves.
The status is visible but easy to misread. A profile marked as an exclusive member of an agency can still be invited and interviewed by a client; the contract that results is simply signed with the agency, at the agency's rate, and the agency decides who does the work.
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.