Movement

Controlling Unit Movement Abilities

For example, Tanya can go into the water, so can seals. Other infantry can't. Some vehicles can also go over water, while other can't. It is not dependant on the locomotor, the MovementZone and SpeedType control this.

MovementZone

MovementZone says where is the unit allowed to go and helps the AI:

Amphibious - ground and water.
AmphibiousDestroyer - can go into water as well as ground. Can destroy infantry obstacles via weaponry. In RA2 this is tied to being an amphibious infantry while in TS it was for the ampibious APC.
Crusher - ground only, can crush infantry, but not armed.
CrusherAll - ground only. Can crush any mobile object, including walls.
Destroyer - ground, no water. Can destroy building obstacles and crush infantry.
Fly - fly anywhere.
None - same as Fly.
Infantry - only clear ground, no water.
InfantryDestroyer - only clear ground, no water. Can destroy obstacles.
Normal - clear ground, no water. Can destroy trees and such obstacles. Can crush infantry.
Water - water only.

SpeedType

SpeedType defines the way units move over certain terrain types. Looking at the rules, we see that each terrain type (Clear, Rough, Road, Tiberium, Water, etc.) has it's own section. Let's look at one:
[Clear]
Foot=100%
Track=100%
Wheel=100% ¹
Float=0%
Hover=50%
Amphibious=80%
FloatBeach=0%
Buildable=yes

The numbers indicate at what percent of it's Speed= can the unit with a certain SpeedType= move over this terrain. You see that units with SpeedType=Hover, for instance, can move only up to half their normal Speed= over this terrain. You can use percent over 100, it'll work fine. The terrain types are as follows:

Clear - Clear ground, no obstacles.
Road - Dirt or paved roads.
Rough - Rougher terrain such as rocky areas or thick grass. In RA2, this defaults to the same settings as Clear
Rock - rocks, trees, cliffs, anything impassable.
Tiberium - areas covered in Ore/Gems/Tiberium.
Water - Water as in rivers or ocean areas.
Railroad - Train tracks (Works in TS, in RA2 requires Terrain_Expansion (aka TX)).
Tunnel - Tunnel Entrance/Exit (Works in TS, in RA2 requires Terrain_Expansion (aka TX)).
Beach - The line where water and ground join.
Weeds - Tiberium Veins (Works in TS, in RA2 it is not assigned to anything and can be used for new movement rules)
Ice - Water gone cold (Works in TS, in RA2 it is not assigned to anything and can be used for new movement rules)

Some units also are restricted to certain terrains, say trains are restricted to railroad, ships are restricted to water, and some spawned aircraft are restricted to water(it affects only their landing). You can restrict other units to some kind of terrain, say to roads only if you so desired. To do so, use MovementRestrictedTo= with one of terrain types described above - Clear, Rough, Road, Water, Tiberium, Rock, Beach...

¹ SpeedType=Track is not used by any object ingame. So you can either restore it for it's original purpose (tracked vehicles) to provide certain variation against standard vehicles (the vehicle locomotor defaults to SpeedType=Wheel) (see also below), or use it for something entirely unrelated, like a class of vehicles that can't go over tiberium for whatever reason.

Tracked vs Wheeled

[General]
TrackedUphill=1.0
TrackedDownhill=1.2
WheeledUphill=1.0
WheeledDownhill=1.2

These are multipliers to units' speed when climbing up/down hill.