This grouping represents the IAM Subjects API page of the documentation. In this instance a subject is an organization that you invite to share access to Assets in your Tenancy.
The IAM Subjects API is used to import the organizations identity to your Tenancy. Their level of access to your Assets is defined by an IAM Policy. This concept is discussed in detail here and here.
When you import an external organization into your tenancy they will provide you with their Subject string. In order to import a Subject using the API, it will need to be decoded to access the wallet_pub_key and tessera_pub_key that are required by the POST call in this group.
The following bash command will do this:
echo $SUBJECT_STRING | base64 -d