I found this piece of information

http://www.posersoftware.com/documentation/12/Poser_Reference_Manual/Tools/Modifying/Saving_Morph_Injection_Poses.htm

"If the morph you created includes a dial in the BODY actor (for a full body morph) it will automatically select the morph in the affected body parts when you check the morph in the BODY actor."

However, it does not answer my question re: difference between Create A FBM and Spawn A FBM

Eternal Hobbyist