authorityIndexof a Transaction account will be used to generate the seeds necessary to perform the CPI. Most often you can simply use the
authorityIndexof 1. For example, the Squads client uses an
authorityIndexof 1 for all default PDAs (the vault, program upgrade authorities, etc).
authorityIndexupon creation, and all instructions attached to the Transaction should use the same authority PDA for any account that is designated as a signer.
authorityIndexof 0 is reserved for internal multisig instructions, and should generally be avoided.
["squad", multisig, authorityIndex, "authority"]