This element is part of multiple samples, e.g. https://learn.microsoft.com/en-us/azure/active-directory-b2c/add-ropc-policy?tabs=app-reg-ga&pivots=b2c-custom-policy
Yet it's not explained anywhere, and it's missing from reference documentation.
It is explained in the Azure B2C custom policy starterpack.
Claims are presented to the Relying Party Application in a token generated by CPIM. However, a Technical Policy may state, using a true or a false for this element, that the original assertion which was returned from the Claims Provider(s) must also be preserved so that if needed, it can be looked at by Relying Party for auditing or diagnostic purposes.