I am confident this has been cleared up prior but I cannot find the proper thread and as such I apologize in advance.

I know the basic way to get RC's by selling gold and up units, preferably those that were summoned. Silvers and fodder units give you nothing.
However I was looking here ( http://aigis.wikia.com/wiki/Trading_Post ) and where it says how to obtain RC's it mentions Silver summoning granting 20 RC's while Gold and up grant 10.

Can someone explain this? The only method I know of is discharging, so what is this summoning business?