I ran across then when browsing a web site for an exhibit here in SF: “This exhibition is supported by an indemnity from the Federal Council on the Arts and the Humanities.”
So I looked it up and apparently the National Endowment for the Arts has an indemnity program that was created to allow US museums to pass the liability (e.g. insure) for the collection they have on loan to the US government. If there is a loss, the institution must pay a deductable and the rest of the loss is covered by the government. What a clever idea. It’s a seemingly cheap way for the US government to encourage museum sharing and reduce the cost of the museum insuring the art work.