Here's the logic behind the icon groupings, that was asked by a few as well:
Move/Fire Toggle
Unit Actions:
- Human Wave, Smoke, Starshell
Unit Handbook (F2)
Combat Actions:
- Arty Dialog, Air Attack, Resolve Assault
Movement Actions:
- Load/Unload, Dig-in, Double Speed, Save APs for Unloading, Save APs for Firing
Show Range Actions
- Command, Visibility, Hard/Soft Attack, Movement
Map Actions / Toggles:
- Thermometer, Unit Bases, Units, Objectives, Contours, Hex Grid, Rotate Map, Jump Map
Highlight Units:
- Anti-Aircraft, Artillery, Mine Clearing, Disrupted, Fixed, Moved, Fired, Low-on-Ammo, Spotted, Leaders, HQs, Organization
Add/Remove Units Dialogs
- Show arrived, Remove Units
Unit Orientations
- Turn Clockwise, Turn Counterclockwise, Bottom of Stack, Top of Stack
Previous/Next Unit
- Show Previous, Show Next
Next Turn
Here's the image from Matrix CS forums. Please note the icons are not final, there's still a couple of them Mike's been further tweaking for additional clarity:
Hope this helps