You mean clamping? It limits the actual texture coordinates to a [0..1] interval. The sky boy uses texture coordinates between [0..1] but the bilinear filtering usually makes the coordinates go slightly below 0 and above 1 (the mentioned "bleeding"). When clamping is disabled (which is default), these coordinates will wrap to the other side of the texture. With clamping enabled, they will be clamped at 0 and 1.
The SkyBox class actually enables clamping for the textures that the sky box uses, but if one changes them afterwards, it can't do it for those textures.