There are two kinds of TP's in the mod. Those with 5km range, and those with 2500 km range. Those with 2500 km range are the traditional landing craft from vanilla hoi. The 2500 KM range craft represent sophisticated sea worthy shallow bottom landing craft capable of doing opposed beech landings.
The problem with vanilla system is that in the real world these kind of craft were not really high-seas capable ships -- they would be easily sunk in even moderately bad wether. You may remember D-day was cancelled because of bad weather partly out of the fears about the difficulties the Landing Craft would have going through the English Channel. They would never be used to transport troops from Florida and then unload them in Morocco, as it is in vanilla.
So the mod has two kinds of unit capable of transporting troops, Troop Transports and Landing Craft.
Troop Transports have a 5km range and can only transport troops from port to port, and Landing Craft are just like the vanilla ones. UK, for example, has only a few of these sophisicated opposed landing craft at the begining of the game, but a whole ton of the 5km ones that can be used to do mass transfers of troops, say from India to Europe or back. The same is true for USA. The Troop Transport represent converted ocean liners, freighters and other ships that were aquired by the navy specifically for long distance high seas transport of troops and equipement.
This is because neither the USA nor UK had a huge opposed landing capability when the war started, and you may remember that the Allies decided to do the Dieppe operation in 1942 specifically to see if it were possible to use their limited opposed landing capabilty to capture a port. A port which could then be used to unload troops and equipment at a dock in a traditional manner. After the failure of this operation, the Allies re-assessed their plan, and decided to build the capacity for a full scale beech landing, where not only the intitial assault, but a good portion of resupply and reinforcement could be done directly to the beechhead.
In vanilla, D-day type operations are available to any of the major naval powers from day one, something which is not historically accurate. In the mod, the players must build the capacity to do large scale opposed beech landings (2500 km tp's) such as D-day until they build it, but they still have the capability (5km TT's) to do mass transport of troops from one theater to another.
