Oh boy, Channel ballz and thingz... ; )

A, B, C & D Texture Channels\Columns all do the same thing.
They determine how texture color, value (amount) and optics (reflects, transparency,
refraction) are applied/combined/scaled to make up a Material.

A Component Texture or any combination of up to 4 textures, (the leetle thumbnails to the
right>top in the mat lab are Texture Components a/k/a *.brt) is used to make up the
*Material* that you see in the Material Preview Window at the top left of the Mat Lab.

An example;
You can use the Diffuse Color from Texture in Channel A
............the Ambient Color from Texture in Channel B
............the Specular Color+ Value from Texture in C
............the Bump and Transparency from Texture in D
The result will be a *combination Material using the features you select from each
separate Texture.*

Mat lab Stuff continued...
You can use any combination of AB&C you wish. IE, multiple bump height info from A B &
C, Combinations of Diffuse colors from different textures, a mix of Ambient Colors...But if
you have something in A and want to combine it with something in C you will have to live
with what is in B also.
Better yet, learn how to tweak the textures (.brt) you don't like in the DTE.
The result will be the *Material* you see in the Preview Window.
D Channel acts independently of the others when used for Value and Optics. When used
in Color it combines any others that are there or uses whatever value it has when applied
alone.
I suppose you could look at it as kind of an overdrive channel. Mostly it's used to make
transparent and heavy bump areas appear on a Material.
It seems optimized for picture/grayscale use.
Don't confuse Material Lab Texture Channels/Columns A,B,C & D with DTE Modes A,
AB, and ABC.

Numeric Entries

Bump Height is one of the few Bryce values you can push past shown limits.

Kristena asked...

<<and I want to give it more bump.. the bump is at 100% in channel C say.. how do I give it
more bump.. if it's at 100% already..>>

[A Direct answer – a miracle!]
Click in the Bump Height box where you see 100.
Type in any number from- 101 to -999 or +101 to +999.
Hit Enter.

Bump Height is one of the few that will accept a number over 100%...
Negative numbers seem to work better for rock, mountains etc. Try -250 to start.

Don't forget to save your material (.mat) by clicking the small arrow to the tight of the main Mat Lab Preview.
Simply select the Preset library of choice and click Add at the bottom left, give the mat a name.
Click the checkmark when done and the .mat will appear in the next available slot.