Multiple choices

[Crossed]

Say I cast a charm. Izzet charm, selesnya charm, dimir charm, etc....

When must I choose which part of the spell is cast? During the cast? Or during reaolution?

[Crossed]

Face palm lol. Of Course it is....

[Gorzo]

Which part of the spell you are choosing to cast is called the "mode." Selecting the mode is an essential part of casting a spell (if applicable), and is a requirement of successfully casting a spell (and adding it to the stack), along with paying costs (mana, typically) and selecting target(s).

Some other instances of cards that choose modes:
{Austere Command} and the other Commands
Split cards and fuse cards such as {Far // Away}
{Soul Manipulation}, "choose one / or both"

Hopefully that makes thing clear, and not more confusing :)