Specification: https://forum.getodk.org/t/create-and-update-entities-from-repeats/55163

The writeup above says only one row will be initially allowed on the entities sheet at a time. That means a form could either be configured to create/update a single entity OR multiple Entities from a repeat but not both (e.g. not a household Entity and multiple Member Entities). There's a chance we'll expand initial scope to include multiple Entity declarations but if so, let's track it in a follow-on issue.