Difference between "metadata" and "custom metadata"?

Some resources in Vault allow you to set “metadata” on them, while others allow you to set “custom metadata” on them. When retrieving data, you can often see both “metadata” and “custom metadata” values.

For instance, you can set metadata on Entities, but custom metadata on Entity Aliases. Groups can have metadata, but Group Aliases have no metadata at all. The difference on KV2 secrets seems to be the most clear: it appears that in this case, “metadata” refers to automatic, system-level information about a particular secret version, whereas “custom metadata” is arbitrary and user-defined.

I’ve tried to find a clear distinction between these two, but have had no luck, apart from what I have inferred above. Can anybody clarify the usecases that each kind of metadata is intended to address? Thanks in advance.