Support reactions are reported as forces acting on the elements connected to the support. The reaction forces are reported with respect to the global coordinate system. Positive support reactions forces act in the same direction as the positive global axes. The sense of positive moments can be determined using the right hand rule.
As an example consider a building column that is supporting gravity load. This gravity load acts in a downward direction. Thus the force imposed on the bottom of the column acts in an upward direction. This is the reaction force reported by ETABS. Because the upward force is in the same direction as the positive global Z-axis, the reaction is reported as a positive value acting in the Z direction.
The figure below illustrates positive support reaction forces.