Since a payment number is required, most people who are recording a single payment will use the voucher number for the check number.
If people are processing a batch of payments at once using the print check routine, they frequently will carve out a series of numbers either below the current check series or in the distant future.  

When printing checks, the proposed check number for that asset account doesn't have to be used.  You can type in a different number than what will flow in when a choice is made in the drop-down.  It must be a number.  No letters are allowed.

FIMS can handle multiple check with the same number.  However, using the same number over and over makes finding checks difficult, so we advise using a different sequence.

If it is necessary to record the actual wire number on the payment record, after posting the payment you can find the ACH or EFT on the payment tab and open it up and type in an alphanumeric number into the Add'l Payment # field.