Featherstep through undergrowth / heavy undergrowth

I searched and didn't find anything on this, so I'll just ask... In reading the Featherstep spell, it lets you move normally through difficult terrain. What exactly does this let you ignore?

"For the duration of this spell, the subject ignores the adverse movement effects of difficult terrain, and can even take 5-foot steps in difficult terrain."

What defines difficult terrain exactly?

"Difficult Terrain: Difficult terrain, such as heavy undergrowth, broken ground, or steep stairs, hampers movement. Each square of difficult terrain counts as 2 squares of movement. Each diagonal move into a difficult terrain square counts as 3 squares. You can't run or charge across difficult terrain.
If you occupy squares with different kinds of terrain, you can move only as fast as the most difficult terrain you occupy will allow.
Flying and incorporeal creatures are not hampered by difficult terrain."

It seems to me that it's the "2 square of movement" rule that specifies that it's difficult terrain. Some terrain is more difficult than others and require 4 squares of movement to go through.
• Undergrowth = 2 squares
• Heavy Undergrowth = 4 squares
• Snow = 2 squares
• Heavy Snow = 4 squares
• Dense Rubble = 2 squares
• Ice Sheet = 2 squares
• Steep Slope = 2 squares
• Steep Stairs = 2 squares
• Many spells cause difficult terrain, which would cost 2 squares as well.

The really strange part of all this is that in the description of difficult terrain, it mentions "heavy undergrowth". So does that mean that even the more extreme types of terrain are also considered "difficult"?

If so, Featherstep should allow unrestricted passage through 6 feet of snow or heavy undergrowth. Is this right?

Now, to further this. Some terrain makes you preform skill checks or adds to the difficulty of them. Is this "difficult terrain"?
• Light Rubble = +2 Acrobatics check DC
• Uneven Flagstone = DC 10 Acrobatics check required to charge/run
• Slipper = +5 Acrobatics DC
• Scree = +2 Acrobatics DC

Does Featherstep let you ignore those checks as well?

OK, what about if Nimble Moves and Acrobatic Steps are added to this? Do those work in 6' of snow?

My catfolk ninja and his group will soon be traveling over the pole . . . is Snow considered "difficult terrain" or does it hamper movement in its own unique way?

Just want to make sure Featherstep would take care of any movement penalties due to snow.

Thanks.

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anyone?

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Difficult terrain is anything that says it is difficult terrain. Generally anything related to the terrain that costs you extra movement is considered difficult terrain (so deep snow qualifies, but darkness does not).

thanks...

im picturing crouching tiger hidden dragon stuff

Clumsy races trudge through deep snow... Elves walk on top of it.