This page is pulled verbatim from the Easy Parallelisation: Killing lots of micro with one stone thread.

Add a modifier key (let's say crtl) to change an order queue from sequential to parallel. Hold this key down and issue a number of orders (n) to a group of m units, and then release the key. All the units selected will split themselves into n sub-groups (as long as m > n, if this is not true, they will go off on their own, and any unit that finds itself idle will be assigned one of the outstanding tasks) and perform one of the orders. The pathfinding loop will automatically assign units to orders on a closest-first basis, but also attempt to ensure equal force composition if possible (give two attack orders to 10 tanks and 5 arty, and it will split them into a group of 5 tanks and 3 arty and a group of 5 tanks and 2 arty).

If there are any units selected for which the order doesn't make sense (units without weapons given an attack order, units without nanolathes given a build/repair/reclaim order, units ordered to attack something they can't hit), they are split up evenly between each sub-group and treat the order as a "Guard" order with that sub-group as a target (meaning it's trivial to take units along for protection) for the duration of the order.

Since these orders, from the PoV of the units assigned them, are no different from normal orders, they can even be queued, with a sequence of clicks of the form: <shift><ctrl>first batch of orders</ctrl><ctrl>second batch of orders</ctrl><shift>, which may be useful if you want to, say, annihilate a selection of high priority targets with bombers, and have any survivors go after some low priority targets. Or split your forces into three and have them follow three winding paths.

This mainly reduces the number of clicks it takes to multitask similar tasks, and links well with area commands (since sometimes one would want area commands to be done in sequence, and sometimes in parallel).

Finally, one could even give orders with no units selected at all, which would be the game's cue to assign the nearest available unit to the task (or perhaps all available units, if you signified priority somehow, perhaps by double-clicking the order instead of single-clicking it, or even a priority-priority (triple click? Some other modifier key?) which will actually pull units of previously assigned orders)). Useful when you don't really care which engineer you get to reclaim that wreck, you just want someone to do it sooner rather than later. Or if you spy a stratbomber lining up to snipe your commander, and you just want everything you've got to shoot at it no matter what else they were doing before.