Help Constrain particles on axis

[Richard]

Constraining an object so that it moves in a certain direction is equivalent to projecting its velocity vector to that direction, which you can do using that function.

In the link I provided, there's an example that constrains a rail-mounted gun so that it only moves along the rail direction. You can do the same for particles, so that they only move in a given axis.

Why the document combine transform and vector like (Transform target, Vector3 railDirection)?

I cannot add rigidbody to particles so that I add constrain on axis?