Keys, other than the main key pair, are the public keys that agents use to verify content from other main Repositories in your environment or from Trellix source sites. Each agent reporting to this server uses the keys in the Other repository public keys list to verify content that originates from other ePO - On-prem servers in your organization, or from Trellix sources.
If an agent downloads content that originated from a source where the agent does not have the appropriate public key, the agent discards the content.
These keys are a new feature, and only agents 4.0 and later are able to use the new protocols.