Below is a tree-representation of an action sequencing hierarchy, taken from Fischmeister et al 2016 (Journal Cortex).

Could anyone explain (the paper does not) by which rationale the different nodes (levels) in the tree are deemed to be either Head or Preparation?
Also, the tree seems ill-formed as the nodes at the second level from the top do not have a title/description, but only their Head/Preparation function specified.