https://github.com/WICG/turtledove/blob/main/FLEDGE.md#6-additional-bids indicates that

The ... topLevelSeller fields are used to prevent replay of this additional bid.

Additional bids don't have a corresponding call to generateBid(), and so the seller and topLevelSeller fields in an additional bid are intended to allow for the same acknowledgement as those in browserSignals.

Also documented as such in the spec - https://wicg.github.io/turtledove/#additional-bids-section.

Originally posted by @rdgordon-index in #10740

As discussed during the WICG call on 02/05/25 - removing the requirement for additional bids requiring toplevelseller to be known in advance would be beneficial vs trying to pass back and forth this information if the buyer doesn't care who the TLS is.

Proposal:
Allow TLS in additional bids to be optional in the JSON blob returned by the buyer, or to support a wildcard if they don't care what the TLS is.

Ref: prebid/Prebid.js#10740 (comment)