Well, let me pull out a bit of historical information on "Garrison Artillery."
In the early ages of Canada, around the time they became their own Dominion, they had this little problem. Canada is really big, and a lot of it is empty space. It's hard to keep a nation going without roads and without infos, so:
The RCMP were formed to keep the law in the empty space, and
Canada's Artillery units were broken into two different groups.
The first, the "Horse Artillery" were sort of a mix between what you get from Civ 5 Lancers and Cannons. They're sort of like the cannon version of the Chariot Archer. Their job was to move quickly throughout the countryside, set up, and blast stuff. Horses dragged cannons, and men on horseback carried "Hand Cannons" and the like.
The second was the Garrison Artillery. Garrison Artillery didn't move around much at all. Instead, their job was to 1) take care of Canadian Forts, and 2) shoot stuff that got too close to forts.
We decided that Horse Artillery are a bit too aggressive for our defensive purposes, so while they're cool, the Garrison side of the Artillery split is a better choice mechanically.