You can create particle expressions from the Expressions editor.

The particle radio buttons let you write two types of expressions: creation and runtime (before or after dynamics calculation). You can use both types for any attribute of a particle shape node.

A creation expression generally executes when you rewind an animation or when a particle is emitted. A runtime expression typically executes for each frame other than the rewind frame or the frame in which a particle is emitted, before or after dynamics evaluation, as specified. By default, either type of expression executes once for each particle in the object.

Creation and runtime expressions don’t execute at the same time. The age of each particle in the object determines whether a runtime expression or creation expression executes. Execution details are in Creation expression execution and Write runtime expressions.

The Default Object, Evaluation, and Convert Units options become dim when you select a particle shape node, and you can’t use them.

Default Object is dim because a particle shape node’s attributes can be controlled by only one creation expression and one runtime expression. The particle shape node is always the default object when it’s the selected object.

Evaluation is dim for particle shape node expressions because it has no effect on particle shape node expressions. See How often an expression executes for more information.

Convert Units is not selectable because you can’t alter how Maya handles unit conversions for particle shape node expressions. See Optimize expressions for details on how Maya converts units for other types of expressions.