How to include list of items into certificate?

Hi!

We want to include list of specific entries into certificate.
Lets say entries using following schema
[
{“id”: 1, “name”: “item 1”},
{“id”: 2, “name”: “item 2”},
{“id”: 3, “name”: “item 3”}
]

Can you please advise what Blockcert property we should use for this?
What is the purpose of badge.signatureLines? Documentation(https://github.com/blockchain-certificates/cert-schema/blob/master/docs/schema-2.md) doesn’t cover this property, but it is used in test certificates.
Or maybe define custom property. Are custom properties allowed by Blockcert standard?

Thank you!