What is the purpose of "parity" in SITOR (NBDP) communications?

The purpose of "parity" in SITOR (Selective Identification of Telex Over Radio), which operates with the Narrow Band Direct Printing (NBDP) mode, is to provide a method for error detection and correction during communication. In this context, parity is used to ensure the integrity of the data being transmitted over High Frequency (HF) radio frequencies.

When employed in two-way Automatic Repeat reQuest (ARQ) exchanges, parity helps to identify transmission errors. If an error is detected, the receiving station can request the sender to retransmit the affected data. This capability is crucial in maintaining reliable communication over HF channels, where noise and other interferences can degrade the quality of the signal.

The reason the other options do not fit is that not all types of broadcasts in SITOR utilize parity in the same way. For instance, the distinctions made in some of the other answers pertain specifically to different types of broadcasts or processing techniques, such as one-way broadcasts that typically do not have an interactive error correction capability like two-way ARQ does.