The CEC system module consists of two main components: mailbox manager and StreetPass communicator. Both work on the system save 00010026, which stores all mailbox information and message data. A simplified diagram of CEC communication model is
Application <=> Mailbox Manager <=> System Save (Mailbox) <=> StreetPass communicator <=> NWM/network <=> StreetPass communicator of another 3DS <=> ...
Many CEC service (cecd:u and cecd:s) functions are interfaces exposed from the mailbox manager, which read and write the system save. The mailbox manager is essentially a thick layer over direct file IO, which performances data initialization and verification on top of raw system data read/write.