I found the post from Rudankort from Aug 2012. This is a copy & paste of the relevant parts:
Usable Transports and Transport Category
These columns are closely related, and together are used in the new transport support mechanics. Here is how it works.
1. Equipment file has two new columns which follow "Exclude from theatre" column. First lists transports the unit can use. Second is transport category. Transports in the list are separated by spaces. So "land sea rail" means that the unit can use land, sea and rail transports. A transport can have several categories too. So for example normal air transport can have "air" category, while glider can have "air glider" categories which means it can transport both infantry and heavy equipment. In general, the rule is: if unit's "usable transports" list and transport's "categories" list have anything in common, this transport can carry the unit. Example: infantry has "land sea air rail" usable transports. Glider has "air glider" categories. They have one item in common: "air". So glider can carry infantry.
2. If the two new columns are left empty, the game assigns them automatically. There are 4 default categories which are hard-coded in the game: "land", "sea", "air" and "rail". By default, all land transports get "land" category, all sea transports - "sea" category etc. Also, all leg and towed units get "land" usable transport, all ground units - "sea" and "rail" usable transports and all infantry units - "air" usable transport. This means that by default the game works exactly as before - but using the new transport system. When filling in new columns, don't forget to specify default categories in the list. Thus, by default Gigant will have "air" category, as long as its category column is empty. This happens because it belongs to air transport class. Now, if you put "glider" in its category column, it will assume this category. "air" will no longer be assigned automatically! If you want it on the Gigant too, you would need to specify "glider air" in its category column.
3. There is a new unit trait called "glider". Units loaded into such transport behave similarly to paratroopers: they can land on any hex, not just airport. But unlike paratropers, the landing occurs straight below the glider and no randomization takes place.
4. Scenario params screen in the editor changed. There are no longer sea/air/rail transport numbers and types on the second tab. Instead, "Transports" tab was added. Here you can select any transports and their numbers, up to five different types. So you can have several normal air transports and gliders at the same time.
5. When embarking a unit, the game will search for the first available transport suitable for this unit. The search is done in the same order in which the transports are listed in the Editor. So if normal air transport comes first, and glider next, the game will use normal air transport by default (for infantry, paratroopers etc.). But if you embark a tank, glider will be used, because normal transport is not usable for tanks. On the contrary, if you put glider first in the list, it will be assigned to everything (tanks, infantry, paratroopers etc.), until we run out of gliders. Then normal transports will be used.
You are free to add new transport categories at will. Just keep in mind that the total number of different transport categories used in the eqp file should not exceed 32. This number includes the 4 default categories (land/sea/air/rail) too.
End of quote
The "glider" & "heavypull" categories are not hard-coded but are assigned just as if you or I modded it. These 2 could be changed or re-named as needed though "glider" seems kind of necessary. Hope that helps. A link to the original thread:
https://www.slitherine.com/forum/viewto ... ry#p342090
