UECP stands for Universal Encoder Communication Protocol.
This is fundamental if you want to use an RDS encoder within a transmitter network.
The specification originated in the EBU in the nineties, but the RDS Forum has updated it several times since then.
The latest version supports all known RDS/RDS2 features.
The RDS Forum no longer publishes this specification as a separate document. Instead, during 2021/22 it became the new Part 10 of the RDS standard IEC 62106.