I just read some Sebastiano testimony and he did not only view Nick SR., Arcadi, Renda and Sollecito as under bosses but he included Giordano and Del Balso. He was later asked in the testimony how are Giordano and Del Balso distinct from the others and it was then that he changed their title from underboss to lieutenants and that they reported to Arcadi. On his power point slide he just lumped everyone as under bosses most likely because they reported to Vito. Sebastiano testimony as to titles was very vague and did not offer to give titles on the other four because he was not asked to do so.
Têtes dirigeantes in French means Heads or Masterminds.