Apply Rest Pose

The Apply Rest Pose button differs from the standard Blender tool in that it also changes the meshes that are parented to the affected armature.

Select the posed Basic wear rig and press Apply Rest Pose. The new foot pose becomes the rest pose.

Sometimes it is necessary to apply the current pose as rest pose before merging for some of the armatures, but not for all.

Import Aiko with Bardot outfit with Mesh Fitting set to one of the Unmorphed options. Then the foot and the sandal have different rest poses. The foot is posed to fit into the sandal, but since the sandal inherits the pose from the foot, the pose is applied twice to it. We fix this by making the foot pose as the character's rest pose. The sandal rig must not be selected, because we don't want to change its rest pose. Nothing moves in the viewport after we use the Apply Rest Pose Tool, but the tip-toe pose is now Aiko's rest pose. When we now Merge Rigs, the sandal jumps into position.