Thread:Stray Bird/@comment-28818979-20161217150031/@comment-27971189-20170703060926

Einzbern, Tokiomi, Kayneth and Zouken use catalyst they used on FZ because they know the result.

Einzbern know its Avalon with its cheat protection and who is the owner. Tokiomi know Gil possess GoB, he even know Gil owns Ea when Gil haven't used it. Kayneth want Iskandar because his potential, while Zouken fully aware what kind of person can be summoned with Arondight that he is OK to let Kariya doing things as he believed the Servant will rely more on skill than magical power.

Yggdmillenia also chose Vlad because he know Vlad got 100% Fame Boost. Vlad bio do note that Vlad got more than just a fame boost if summoned and fight in Romania.

I'd say such knowledge come from them getting involved with the system. Basically, they are the game designers, they must be aware who is the strongest character in game accessible to them, especially the three families. Don't put them on same class as Shirou, Rin and Sakura.