The Fixed Constraint node adds a condition that makes the entire rigid domain fixed (fully constrained). The displacements and rotations are zero in all directions. There are no settings for this node. See Rigid Domain.

Select Evaluate reaction forces to compute the reaction force caused by the constriant. The default is to not compute the reaction force. When selected, the constraint is implemented as a weak constraint.

Select Apply reaction only on rigid body variables to use a unidirectional constraint for enforcing the prescribed motion. The default is that bidirectional constraints are used. For a fixed constraint, there is no effect of changing this setting.

Physics tab with Rigid Domain node selected in the model tree: