The start of the turn after air supply, air strikes and assigning specialist steps. Specialist steps give a unit some sort of bonus, either through a special ability like removing entrenchments, adding artillery shifts (there are no on-map artillery units) and as long as a unit is in supply, the specialist step will become active next turn. Each unit can have 1 specialist step assigned to them, although there is a limit to the amount of specialist steps that can be bought/are available in a scenario.
The Tank corps in the north was hit hard, and it now only has 1 regular step and its towed AT step. Towed equipment steps become suppressed after moving. Those purple dots some Soviet units have indicate artillery steps, and those are usually towed, so as long as they don't start their turn next to one of my units, they can't attack using that step.
Units generally recover their suppressed steps normally over time as long as they're in supply, with veteran and elite units recovering more suppressed steps per turn.
I've given 1 infantry division and 4 Panzer divisions an engineer step. Engineers remove entrenchment and also remove the negative shift for attacking across a river, aside from providing additional attack/defence values.
Contrary to games, like SSG games, where each step represents an abstracted amount of efficiency that scales up with the number of steps (so each step doesn't necessarily represent a certain fixed value), in Unity of Command each step has the same attack/defence value for a unit.
Air strike targets have a yellow circle around them and the units that got engineer steps have a blue circle around them. One of the units is not visible on the screenshot.
