Modules A-130 and A-131 provide voltage-controlled amplification. For audio signals, you would normally use the exponential VCA (A-131), and for control voltages, the linear VCA (A-130). It doesn't always have to be that way, though.
The amount of amplification the VCAs provide is determined by the voltage at the CV input, and the position of the gain control, which sets the overall gain in the system.
The VCA has two audio inputs, each with an attenuator. They are amplified by an amount determined by the combination of the gain and the two CV controls.

For more detailed information please look at the English user's manual A1301_man.pdf

Remarks:
Over the years three different versions of the module A-130 und A-131 have been manufactured:
Version 1:
The first versions of A-130 and A-131 were equipped with the Curtis chips CEM3381/PA381 (A-130) and CEM3382 (A-131). These versions of A-130/131 were not able to process slowly varying control voltages as they had AC coupled signal in/outputs. They were manufactured until about 2001.
Version 2:
The second versions of A-130 and A-131 were equipped with the OTA circuit CA3080 and they were able to process even slowly varying control voltages as they had DC coupled signal in/outputs. They were manufactured from 2001 until March 2013.
Version 3:
Die latest versions of von A-130 and 131 are equipped with the high-end VCA circuits SSM2164/V2164 (A-130) and THAT2159/V2159/SA2159 (A-131). Even they are able to process slowly varying control voltages as they have DC coupled signal in/outputs. These are manufactured since March 2013. Unfortunately it's not possible to upgrade older modules because the new circuitry is totally different.