Concurrent Forces

A set of point forces is considered concurrent if all the lines of action of those forces all come together at a single point.

Concurrent Forces
Because the lines of action for the gravitational force and the two tension forces line up at a single point, these forces are considered concurrent.
Non-Concurrent Forces
Because the lines of action of the gravitational force and the two normal forces do not intersect at a single point, these forces are not considered concurrent.

Because the forces all act through a single point, there are no moments about this point. Because no moments exist, we can treat this body as a particle. In fact, because real particles only exist in theory, most particle analysis is actually applied to extended bodies with concurrent forces acting on them.