SPS 2000 and Dallas Keys

Hi - can someone please clarify the following. We're thinking of using Dallas keys to get the staff to sign on the SAM4 SPS-2000 we have however dont know if I should be ordering Magnetic or non-magnetic keys.

Can anyone advise me what I need and if the Key needs to be kept on the reader whilst the sale is made or can it be just pressed against to sign-in then removed for the order to be made. We need the ability for staff to have the clerk interrupt function how ever the manual sign-on is a bit time consuming.

Thanks