nParticle per-particle attributes
nParticle objects are created with internal per-particle attribute ramps for Radius, Color, Opacity, and Incandescence, which are displayed in the nParticleShape node Attribute Editor. The internal ramp attributes affect outputs on the nParticle node, which are automatically connected to the appropriate per-particle attribute on the same nParticle node. This connection is not typical, in that it occurs on the same node rather than between different nodes. These connections are created when an input attribute is selected from the ramps in Scale Input list.
For example, on the Radius Scale ramp, when you a Radius Scale Input, such as Normalized Age, the dynamic per-particle attribute is updated, and the input and output connections are made. If you select Off from the Radius Scale Input, the connections between the input and the output are broken, and if the output attribute is a dynamic attribute, it will be deleted. Be aware that the dynamic attribute is deleted even if it was originally created using the some other method. You can view the Per Particle (Array) Attributes to verify which internal per particle ramps are currently being used by the selected nParticleShape node.
If you want to control nParticle Radius, Opacity, and Incandescence using an expression, you must first delete them by setting the Input Scale attribute on the respective ramps to Off.
The internal per particle attribute ramps work similar to the way that other Maya ramp editors do, and they have similar controls such as Interpolation options and input selections. nParticle internal ramps have additional functionality that allow you to set Input Max values and randomizing scale multipliers.