Markers, which are implemented in Chrono in the ChMarker class, are auxiliary frames that can be attached to rigid bodies.

Markers are coordinate systems that inherit from ChFrameMoving because in the most general case one can also assign a motion to a marker with respect to the owner body
Markers can be used to get the position/velocity/acceleration of a given reference frame attached to a rigid body
They can be used to build mechanical constraints via ChLink objects by using a couple of ChMarker objects from two bodies

Example:

auto marker_c = std::make_shared<ChMarker>();
 
marker_c->Impose_Abs_Coord(X_ca); // or.. marker_c->Impose_Rel_Coord(X_cb);
 
body_b->AddMarker(marker_c);