The V.35 interface was originally specified by CCITT as an interface for 48kbps line transmissions.
It has been adopted for all line speeds above 20kbps, and seems to have acquired a life of its own.
It was discontinued by CCITT in 1988, and replaced by recommendations V.10 and V.11.
V.35 is a mixture of balanced (like RS422) and common earth (like RS232) signal interfaces.
The control lines including DTR, DSR. DCD, RTS and CTS are single wire common earth interfaces, functionally compatible with RS-232 level signals. The data and clock signals are balanced, RS-422-like signals.
The control signals in V.35 are common earth single wire interfaces because these signal levels are mostly constant or vary at low frequencies. The high-frequency data and clock signals are carried by balanced lines. Thus single wires are used for the low frequencies for which they are adequate, while balanced pairs are used for the high-frequency data and clock signals.