since we expand the API to handle replies, we need the reply in the sub
claim. additionally, if we want to move away from using the 'txn' claim
and use the 'sub' claim for cases, it must be possible to specify them
in the sub claim. thats why case:<id> was added.

we either use Zustelldienst or (here), since its a technical specifier
the word "system"

The $schema attribute defines which schema versions are compatible with this schema. Software systems that validate a SET payload against this schema should not only accept SET payloads which exactly match this schema version but also accept SET payloads that validate against compatible schema versions (same major version)