The summoning mechanic has been introduced by the general's handbook 2018
Basically, we can summon units for "free" with a Slann on the battlefield.
a Slann can cast normally up to 3 spells each turn.
if you forego to cast a spell, you gain 3 conjuration points. Obviously, if you forego all 3 spells, you accumulate 9 conjuration points... 10 if the Slann is the general.
These points can be increased by other things (Astrolith bearer, endless spells and so on).
For example:
with 6 points you can summon 10 skinks,
with 12 points you can summon 3 ripperdactyls,
with 24 points you can summon a Oldblood on Carnosaur