Slitherine

Are flank marches explained in the rules?

Are they totally random? How many units can be included? Any type of unit?

Is there a particular range of turns the flank march appears?

They aren't explained in detail because they are supposed to be unpredictable.

The program tells you at the start of the game how many units you have on a flank march, and which flank they are on.

They start testing to come on after a certain number of turns, and keep testing until they come on or the game ends.

Think of them as allies or late arriving reinforcements arriving on a flank, rather than troops that you have deliberately sent on a flank march. You don't control what actually turns up or when.

Your plans are therefore based on the hope that worthwhile forces will arrive, but no certainty of exactly their numbers or when they will arrive.