When a manikin segment is fixed, the fixed segment maintains the same position, and remains unmovable in relation to the references (rigid body) against which the manikin was placed. When you try to drag a fixed segment, it is automatically unfixed while you drag it and fixed again when the movement is finished. When you drag another segment, the fixed segment does not participate in the movement and remains fixed.

When a segment that is adjacent to a fixed segment is being dragged, and it reaches its motion limit in relation to the fixed segment, it will stop.

A fixed segment can prevent other segments from fully performing a Reach. For example, if you fix the left upper arm, and you want the manikin to reach with the left hand, only the lower left arm below the elbow can reach, because the kinematic chain is limited.

When you apply a posture to a manikin that contains fixed segments, the manikin will be repositioned in the new posture, and then the fix constraint is automatically re-applied.

When one manikin replaces another manikin, any segments that are fixed will remain fixed in the new manikin.

You can fix a segment that contains fully or partially locked DOFs (degrees of freedom). When you fix a locked or semi locked segment, it removes the locked state of the DOFs of the segment.

You cannot lock the DOFs of a fixed segment. You have to unfix the segment first, and then you will be able to lock the DOFs.