These options appear as a subset of the Tubes section in the Paint Effects Brush Settings window, and the Attribute Editor for any brush node.
You can map tube length to color, transparency, incandescence, tube width, and split length value ranges so that every tube uses the entire range, or only the longest tubes use the entire range.
Affects how much of the color range (between Color1 and Color2) is used. If this is set to Length, then every tube uses the whole range. If this is set to maxLength, then only the longest tubes use the whole range. Shorter tubes start at Color 1, but end before they make it to Color 2.
Affects how much of the transparency range (between Transparency1 and Transparency2) is used. If this is set to Length, then every tube uses the whole range. If this is set to maxLength, then only the longest tubes uses the whole range. Shorter tubes start at Transparency 1, but end before they make it to Transparency 2.
Affects how much of the color range (between Incandescence 1 and Incandescence 2) is used. If this is set to Length, then every tube uses the whole range. If this is set to maxLength, then only the longest tubes use the whole range. Shorter tubes start at Incandescence 1, but end before they make it to Incandescence 2.
Affects how much of the width range (between Tube Width1 and Tube Width2) is used. If this is set to Length, then every tube uses the whole range. If this is set to maxLength, then only the longest tubes uses the whole range. Shorter tubes start at Tube Width1, but end before they make it to Tube Width2.
If this is set to Length, then every tube splits the number of times specified in Split Max Depth. If this is set to maxLength, only the longest tubes split that often. Shorter tubes split a fewer number of times.