Unfortunately, what Danny said is not quite true. The Keyset URL is not only used by the Content Selection tool. In fact it was first used for grade pass back. WeBWorK's site key is used to sign any LTI Advantage request, and the LMS will request the public key from WeBWorK anytime that it needs to decode one of these requests. The first implemented instance of that was for grade pass back. Content selection was added later.
WeBWorK Main Forum
Brightspace LTI Error: "Keyset URL cannot be reached"
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