From Michael Thierens: 

The use of the term 'Army' is rather misleading. The highest unit in the Canadian army was the Corps, or in full Army Corps. Armies, consisting of at least 4 Corps were always led by the Imperials. There were five British Armies in France.

So we have 1 Canadian (Army) Corps, consisting of 4 Divisions, which each consisted of a number of Brigades. Each Brigade consisted of a number of Battalions, which had Companies, which had Platoons, which had Sections.

One must always remember that the Canadian Divisions and lower units were to a certain extent interchangeable with British units, depending on the necessities of a certain battle etc. This makes it very difficult to define what an ORBAT is. It changed depending on the circumstances, the battle fought, the way a battle developed etc.

Basically you could construct an Orbat for the beginning of the war, for each Canadian Division when it was formed, for each Brigade, for each Battalion and even for each Company or Platoon. Ditto for each year, month, week of even day of the year. That is, if you define Orbat as a means to analyse the composition and strength of a fighting unit. The question is, is this sensible when thinking in terms of the purpose of the Matrix?

A completely different approach is, if the reason for adding Orbats to the Matrix is to show who was in charge of what unit and what were the names and service numbers of the soldiers who belonged to that particular unit. A tremendous job as the members of a particular unit were killed and wounded all the time and so the names kept on changing. You would have to make a roll of honour for each unit which shows all the permutations within that unit over a 4 year period. Hm, interesting challenge?

From Bill Alexander:

As the string indicates it is difficult to define an orbat precisely. Orbats are used to describe the components of a military force, unit or group at a particular point in time and some may argue at a particular place.

For example, the order of battle of the Canadian army is used in one sense to describe all of the units that are on the army list (and some not on the list) at that particular time. It is usually done by seniority. (Sorry don't want to throw another curve at you, but this is important.) Seniority is used to determine who came first. So, the 1st Division is senior to the 2nd. The units in the division are also organized by seniority, so the 1st Bde is senior to the 2nd etc. Within the division and the brigade the units may be changed. (Eg the 60th Bn being replaced by the 116th in the 9th Bde. The RCR was somewhat distraught at being in the 3rd Div when they were the senior regiment of the Canadian army. In WW2 that situation did not occur again.)

So the answer to your question is yes, orbats exist on different levels. And, yes it can describe all the forces of the nation, or the elements of a battalion. On the bottom level the battalion is the basic building block of an orbat, but there may be smaller parts of units assigned. In WW2, many battle groups were made up of a battalion with maybe a squadron of armour, and maybe some heavy support weapons like mg's or mortars. In WW1, the infantry battalion was the starting point for most formations, and didn't divide into smaller units.

For the Canadians in WW1, the largest unit was the corps. The Canadian Corps was shifted around from army to army, but due to Currie's insistence it was not broken up and dispersed into other formations, especially during the German spring offensive of 1918.