Depending on the number of spine bones defined in a motion source and its target character, the way animation is mapped from spine to spine can differ.

When you retarget animation from one character to another, and the target and source characters have the same number of spine bones, the animation is mapped directly from spine bone to spine bone. The source spine animation is reproduced exactly on the target spine.

If the source character has fewer spine bones than the target character, the source spine animation is solved evenly over all the target spine bones. The reproduced animation is not exactly the same, but is adjusted so the spine still moves naturally based on the source character.

This same behavior is available when targeting animation from an Actor to a character. Actor assets have two spine bones, while characters may have many. When you use an Actor as the motion source for a character, the spine animation is mapped directly if the character also has two spine bones. If the character has more than two spine bones, the animation is solved evenly over all the spine bones.