The spec is agnostic on the Assertion Event - it can literally be any other event of any kind. It's covered on the nostrhub link above.
The Assertion Event, to be useful, should be domain specific and as detailed as necessary such that a valid/invalid assesment can be made via an Attestation.
In your case directly above, you're not signing the assertion - that is the place event itself published by the merchant.
You would either be attesting to it directly, or attesting to the other attestations as you trust them implicitly. Both are fine.
The Assertion Event, to be useful, should be domain specific and as detailed as necessary such that a valid/invalid assesment can be made via an Attestation.
In your case directly above, you're not signing the assertion - that is the place event itself published by the merchant.
You would either be attesting to it directly, or attesting to the other attestations as you trust them implicitly. Both are fine.