Muscle Bulge Gradient
The Gradient settings for muscles are listed here.
Weight Map
A weight map painted on the mesh can control how much points are affected by the bone's displacement.
Bend
The amount of displacement is controlled by the rotation of the bone. For example, this can be used with the Blending Mode Multiply to fade another bone displacement layer as the bone goes from 0 to 90 degrees.
Bulge Falloff
The gradient is controlled by the bone's falloff, so where the bulge is completely gone (at either end of the bone) the gradient input is 1. It's basically the inverse of Bulge, below.
Bulge
The gradient is dependent on the distance to the bone's center along its length. At either end the gradient's input is 0, at the bone's center it's 1.
Distance from Bone
The distance of the point affected from the bone drives the gradient.
X Distance from Bone/Y Distance from Bone
As above, but only using the X and Y distances to the point.
Norm.Distance along Bone
The gradient's 0 point is taken as being at the bone's start, and the gradient's 1 point is taken as being at the bone's tip.
Angle from Inside
The angle from the Y-Z plane of the bone drives the gradient. Thus a gradient going from 1 at -180 to 0 at +180 will start out making the mesh fat at the back, then getting slimmer as you move around the bone, till you get no displacement at the other side of the back, leaving a crease where they meet. A gradient with 0 at -180/180/0 and 1 at -90 and +90 will bulge out the sides of the mesh around the bone. (Note that because a gradient here can only displace in X, Y or Z, you'll need to add two gradients to see 2 dimensional displacement)