Does the alert only serve the following purposes:
while its content DO NOT matter. Only the presence of the field and its attributes matter.
The only time its content matter are when the receiving device is an Apple Watch.
Is that correct?
The alert "field" (it is actually a JSON key containing a title, body, and sound keys) is used to alert a person about a critical Live Activity update. When included in the payload, it will light up their device (if appropriate) and display the expanded presentation on devices that support the Dynamic Island or a banner on devices that don’t support it.
Its content DOES matter, as without any text or sound, it is basically a silent notification, and its behavior in the context of live activities is undefined.