When the line is Across The set, the dancers straddle the Centre line, Facing Up or Down.
When the line is on the Side, the dancers are in the Side lines, Facing Across The set.
The Strathspey setting step involves substantial movement to the right. When another dancer is Standing to the right of the dancer who is Setting (as, for example, in 1st Place on the Ladies' Side of a Longwise set with the dancer in 2nd Place), there is a risk of collision, especially in a crowded ballroom; the line of the Setting step should be modified slightly so that the Setting dancer passes just in front of the Standing dancer on bar 1 and returns to the Starting Position on bar 2.
This problem could also arise with the Highland schottische setting step but never with Pas-de-basque, of course, because there should be no sideways movement in this Figure.