Base Form
These settings define the original particle grid that can be affected by various maps and fields. You have control over the size, particle density, position, and rotation of the base form in 3D space. The specifics of the various forms and the controls are explained below.
Base Form
There are three settings that you can choose to use as your form's starting point: a grid of particles, a dense set of horizontal strings, and spheres. All of these base shapes can have multiple iterations by setting the Particles in Z value to a number greater than 1.

Box - Grid | Box - Strings | Sphere - Layered

Size X, Y and Z
Sets the size in X, Y and Z axes correspondingly. The X and Y dimensions correspond to the size of the form horizontally and vertically, while the Z value determines its depth. The Particles in Z and the Size Z work together to control the density of the overall form.
Particles in X, Y and Z
Sets the number of particles used to generate the form along the X, Y and Z axes. Render times are directly proportional to the number of particles. Often a particle density of only a few hundred is needed to clearly define a form. Be careful of high Z values, because the Z value essentially replicates the grid, strings, or sphere, and can cause very slow rendering. Typically, you only need a Z value of 1 to create an interesting surface shape.
Center X, Y and Z
Positions the form in space. Note that this positioning takes place before the different distortion fields (Fractal Field and Spherical Field) are applied, so the controls here in effect move the form through the fields. If you'd rather move the whole thing with fields applied, use the Offset settings in the Transform World controls instead.
X, Y and Z Rotation
These values let you rotate the form in 3D space. Note that this rotation takes place before the distortion fields (Fractal Field and Spherical Field) are applied, so the controls here in effect rotate the form through the fields. If you'd rather rotate the whole thing with fields applied, use the Rotation settings in the Transform World controls instead.
String Settings
When you choose the Base Form Box - Strings setting from the Base Form pop-up, these settings are enabled:
Density
Sets the density of the strings. Higher settings produce denser strings and take longer to render. The default value of 15 will generally produce good results, while values lower than 10 will cause the strings to degrade into dots. Very high values in the hundreds will cause dense glowing lines because of the default Add transfer mode used to composite the Particles.
Size Random
Controls size randomness for the strings so that some strings are thicker and some thinner by varying the size of the particles that make up the string.
Size Rnd Distribution
Controls the (random) distribution of smaller and thicker strings, and generates lines that appear randomly thicker or thinner across the form.
Taper Size
Allows for tapering string thickness at the ends of the strings with two settings: Smooth and Linear. The Smooth option offers a taper that starts closer to the center of the form and the falloff is more gradual. Linear produces a linear falloff model and will cause the taper to only begin near the edges of the form.
Taper Opacity
Allows for tapering opacity at the ends of the strings by offering two options, Smooth and Linear. Like Taper Size, these controls produce slightly different visuals, with Smooth causing the ends to appear shorter and more transparent and Linear to only taper the opacity close to the edge of the form.