Vectors represent the direction and magnitude of a force. As particle systems inherently deal with reactions to forces, it is often necessary to define them in POPs. You can specify vector POPs in one of two ways: Vector or Component form.
Vector definition lets you use a mathematical expression such as:
vector3($VX, $VY, $VZ) * rotate(10,"X")
The advantage of using a mathematical expression for vector definition is that you can animate the vector using standard mathematical functions. In the above example for instance, the vector will rotate about the X axis 10 degrees. Most vector manipulations are easier to state as a vector expression. It would be difficult to express the rotation above in component form.
When specifying vectors in component form, you define the vector by specifying an endpoint with X, Y, Z coordinates, with the starting point of the vector being 0,0,0.
Component form is useful when setting the vector to a constant value or when selectively changing only a few components.