@ petro
may b it should refer the strength of Espada,
top 4 must have to take test to become espada i.e (0 0r 10), primera(1), 2 and 3.
remaining ones jus can be selected by their experiences and matter they know about Arrancar or specially on Espada's powers and abilities.
one can challange the higher rank espada with the permission of owner or lord of espada i.e is u

to gain higher number.
if the challanger win he will get the higher number and will remain at tht number atleast 1 month to challange more higher rank if he wish.
if he fail he can't challange another espada for atleast 1 month.
plz quickly select the espada then the new selected fraccion will be under them, as the joining form should include in whose fraccion they want to join. as its only possible when there will be Espadas.