Why transfer_from cannot be a PDA?

[yurushao]

Hi there,

I was curious why there's a constraint on transfer_from account that the owner must be a system program. This means PDA won't work. Thanks.

liquid-staking-program/programs/marinade-finance/src/instructions/user/deposit.rs

Lines 60 to 64 in 2614737

	#[account(
	mut,
	owner = system_program::ID
	)]
	pub transfer_from: Signer<'info>,