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?
Face palm lol. Of Course it is....
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 :)