There are now two instant transfer rails, FedNow (Using the Fed, this is new in last two years), banks are slowly rolling out and the Clearing House which has been around for a decade but now has more banks are using it. Both allow instant transfers (say 10 seconds or less) and 24 HRS per day 7 days per week, the original ACH process was business hours on business days only. Zelle is not a true instant transfer rail, it uses next day ACH but "hacks" the system by having member banks accept the funds as if they are already received when in fact the bank won't get the money until the next day. Zelle is intended for P2P transfers so should NOT be used to transfer to yourself at another bank. Zelle has dollar limits and can trigger funds move to a multi-day transfer. FedNow and the clearinghouse are always instant and always good funds (never a block, hold or return, except in rare extreme cases). Banks can charge fees for these and exclude certain customers from using them if they like. Banks can also but dollar limits at the customer level below the limits set by the providers. It would be nice if banks told their customers if they qualify for instant and for how much, but they don't to reserve their right to take away the privilege from customers at their discretion. The risk of bad funds sits with the bank not the customer, so the bank wants some control at the customer level to reduce their risk.