There is skirmishing and there is open order. One is an activity and the other a formation.

Obviously skirmishing was done in loose order and therefore relied upon experienced men who could function thusly without routing or deserting. It is this experience that endowed the flank companies with their superior combat power. Light infantry and grenadiers were often kept at full strength by culling the best men from the line companies. The higher proportion of experienced men made these companies better in every respect to their line counterparts. They generally had fewer cowards and more fighters.

Open order (without regard to skirmishing) could be adopted by any formation that was required to maintain a position while under fire. It spreads the men out so as to loose fewer to artillery and what not. The men may not have been expected to do much in open order but it did help if they couldn’t move out of harms way and instead had to stand and "have a taste of it".