Movement

Controlling Unit Movement Abilities (by DCoder)

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:

Amphibious - ground and water.
AmphibiousDestroyer - can go into water as well as ground, but can't crush. Can destroy infantry obstacles via weaponry.
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 - duh, roads.
Rough - area covered in rubble and such minor obstacles.
Rock - rocks, trees, cliffs, anything impassable.
Tiberium - areas covered in Ore/Gems/Tiberium.
Water - duh, water.
Railroad - train tracks (Works in TS, in RA2 requires Terrain_Expansion).
Tunnel - tunnels (Works in TS, in RA2 requires Terrain_Expansion).
Beach - the line where water and ground join.

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 (although the point in that escapes me). To do so, use MovementRestrictedTo= with one of terrain types described above - Clear, Rough, Road, Water, Tiberium, Rock, Beach...

¹ SpeedType=Wheel is not used by any object ingame. So you can either restore it for it's original purpose (light vehicles) to provide certain variation against tracked vehicles (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.