My understanding about constraint force is that it is a force which limits the geometry of particle's motion. For example, situations such as the particle trapped in a track or limited in domain can be assumed constraint force.
But in this point of view, I couldn't understand why friction is constraint force. In lagrangian formulation, we divide forces into two part, $F= F(applied) + F(constraint)$.
If particle moves in one dimension, and assuming there exists sliding friction, that particle can move anywhere. The sliding friction never restrict the domain that particle can move. so I think the sliding force is applied force, rather than constraint force.
Can anyone clarify why friction is constraint force?