Now onto my topic...Evidently, while playing Fate, there is a Summoning Hierarchy when a Master summons their Servant. It's partially explained, but how does it actually work?
We also know that using a Catalyst during the ritual improves the Master's likelihood of summoning their intended Servant - as shown when Kiritsugu used Avalon to summon Arturia as Saber and when Bazzett used Cu Chulainn's earrings to summon him as Lancer. With Rin, she tried summoning a Saber Class - not necessarily Arturia - without an appropriate Catalyst, and ended up with Emiya as Archer because he had the jewel she used when reviving him during his timeline - and since she still had that jewel at the time, it could be considered a Catalyst, despite not being used during the ritual.
One of two explanations can be used as to why a Saber Class wasn't summoned during Rin's attempt - 1) The Class was already filled because, I think, Kiritsugu sealed Arturia in Shirou's shed and so she never "left the War", or 2) Shirou, being in possession of Avalon, increased his chances of summoning Arturia as Saber far more than Rin's chances without one.
Regardless of your preference in explanation, the issue is the Hierarchy. What if two people had a Catalyst for a specific Hero and wanted them either for the same or different Classes. Let's use Cu Chulainn as an example for this, while hypothesising that a second Master had a Catalyst of his like a lock of hair or a piece of Gae Bolg.
Cu Chulainn, by his own admission, could have been one of three Servant Classes - Lancer for his fame in using Lances and Gae Bolg; Berserker for his fame in going into Fighting Frenzies and Caster with his talents in Magic - I'm pretty sure that in the UBW route he says something like how Magic was his real specialty, despite his skill with the Lance, before setting fire to Einzbern Castle.
We have Bazzett attempting to summon him as Lancer with his earring and a hypothetical second Master using a piece of Gae Bolg/lock of hair to also summon him as Lancer - who would be successful? Also, what if this second Master wanted Cu Chulainn to instead fill the Caster Class, had Medea not already been summoned - which Class would he have likely filled?
Also, since Rin has her jewel and Emiya will always have her jewel, does that mean that no matter what timeline/reality, provided that she always develops a crush on Shirou and revives him when he dies, she is destined to summon Emiya as Archer or Saber - perhaps even Caster, despite his poor abilities as a Mage - depending on which Classes are available?
Would love to hear your thoughts on the matter )