Intellectual property on Upwork is the question of who owns the work a freelancer or agency produces, which Upwork's Optional Service Contract Terms answer by default by passing ownership to the client on payment.
Under the default terms the freelancer creates the work, keeps ownership while it is unpaid, and assigns it to the client on payment; the client gets the deliverable and the freelancer keeps the pre-existing tools, libraries and methods they brought, which the client may use as part of it. Payment is the hinge. On a fixed-price contract that is the release of the milestone from escrow; on an hourly contract it is the weekly billing that the client does not dispute. Until then the client has a file and no title to it, which is why a freelancer may withhold source until a milestone is released and a client should not use work they have refused to pay for.
The parties can write their own terms, and larger clients usually do, with a work-for-hire clause, a broader assignment or a licence instead of an assignment. Upwork owns none of it, takes no position beyond the default terms and does not arbitrate ownership; its process concerns payment. Two cases need care: a freelancer who wants to show the work in a portfolio needs the client's permission once ownership has passed, especially under an NDA, and an agency should be sure that its own arrangement with its members gives it the rights it is assigning on to the client.
Copyright, moral rights and the enforceability of an assignment differ between countries, and the default terms are a contract, not a substitute for the law where the parties are.
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.