The A-170 Dual Slew Limiter consists of two separate slew limiters, also known as portamento controllers or integrators.
In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit. Just like a Glide circuitry, it flattens or smoothes the incoming signal.
The top section of the module is the 'simplest' of the two Slew Limiters, with one joint control, which sets both rise and fall times (from approx. 0 ...10 sec).
For precision voltage control this one is recommended, but version One (before May 2009 / check your version number on the module's PCB) had a small voltage drop that was caused by the protection resistor at the A-170 output.
( check Doepfer's solutions for that HERE )
A-170 modules after May 2009 don't have this voltage loss.
The second Slew Limiter has separate controls for rise and fall times, and you can set the overall range (high/low/medium) for these parameters with a three-position switch.
Because the diodes in the lower Slew Limiter are voltage-dependant, there is a voltage off-set of around 0.5 V. This naturally makes it unsuitable for precise 1Volt- per-octave work, but for simple voltage control this is no problem at all.
Not only is the lower Slew Generator suitable for pitch-CV's, you can alternatively use this one as a simple AR (attack/release) envelope generator (works with any gate/trigger or LFO).
Very useful if you don't have enough envelope generators and want some punchy or percussive basslines.
Also worth reading: Wikipedia Current Limiting