PBSA team has extended the permission structure of the Peerplays blockchain. Graphene Chains like Hive uses staic, predefined permissions like OWNER, ACTIVE, POSTING and MEMO. The latest member in the family, EOS had extended the feature to allow more permissions.
Some of the non-graphene chains like Stellar also has extensive permission system to support various use cases.
Authorities and Transaction verification
When a signed transaction is received, the blockchain verifies the signature for validity. Lets say, if a transfer transaction is received, the logic within the chain evaluators checks whether the correct key with necessary authority is used.
Custom permissions
For new custom permissions, new named objects representing permissions are created. They are not directly mapped to accounts. But custom authorities are created and then they are in-turn mapped with the accounts. These changes are also done ensuring backward compatibility.
LLD is available here https://peerplays.gitbook.io/community-project-docs/blockchain/rbac/role-based-custom-permissions-lld
Code https://github.com/peerplays-network/peerplays/pull/365