Module A-138 is a four channel mixer, which can be used with either control voltages or audio signals. Each of the four inputs has an attenuator, and there's a master attenuator, so that the mixer can be used at the end of the audio chain - ie. it can be used to interface directly with an external mixer, amplifier, etc.
The module can be supplied in two versions:
A-138 a: potentiometers with linear response, so especially suitable for control voltage mixing.
A-138 b: potentiometers with logarithmic response, so especially suitable for audio signal mixing.