The "Check" field in

Type the customer's 
check number here. 

Enter the check number you wish to use for the check or draft. The customer can use the next number in their check book or simply allow you to enter 9999 for the check number and it will be processed as a starter check.

The check number is usually located in the upper left corner of the check, and is also printed on the MICR line.

You must enter a check number or the check will not be processed properly.