I use a KeyLab88mk3 with an AstroLab 61, but it would be the same with an AstroLab 37.
The KeyLab is powered by the AstroLab (via the USB-A host port of AstroLab).
The KeyLabs provides the same controls as in Analog Lab:
the encoders have the same functions as the AstroLab encoders (4 Macros and 4 FX levels)
the 9 faders match the 9 “virtual faders” in Analog Lab and the same functions are available in AstroLab.
Typically, for the virtual analog synths, those faders are pre-mapped to the 2 envelopes (Amp and Filter), and for modeled organs such as the B-3 or Vox, they are pre-mapped as drawbars.
For 2-Part Multi instruments, the faders are pre-mapped as Part1/Part2 mix functions (Levels, Pans, FX sends).
They have other pre-mapped functions for digital synths and for physical modeled instruments.
These are the default fader functions, but can also be custom mapped individually for each user preset (with the licenses for the instruments you want to re-map).
Other MIDI controllable features of AstroLab can also be assigned to other KeyLab controls.
Personally, I use some of the pads (4 banks of 12 on the KeyLab mk3) to send Program Change messages to select specific Playlists and Songs, and I use the 4 “soft buttons” under the display to go to Previous/Next Song and Previous/Next Preset.
I also use one of the pads as “Tap Tempo” for AstroLab (CC#110).
Other available AstroLab features include Rotary Speaker Slow/Fast, ARP On/Off, ARP Hold, …
Speaking of the KeyLab pads, they can also be used to send MIDI notes, with velocity and poly-pressure. Instruments in AstroLab that support Aftertouch can also receive poly-AT (like the CS-80, for instance).
Of course, you can also use the Expression pedal input (and the Aux pedals) from KeyLab to control AstroLab 37 (AstroLab 61 has its own Expression pedal input)."
Finally, if you create custom 2-Part Multis in Analog Lab, you can assign one Part to the KeyLab’s keyboard and the other Part to the AstroLab’s keyboard, for instance the piano/electric piano part played from the KeyLab88 and bass/leads/pads played on the AstroLab keyboard.