There are provided an image forming apparatus, an image forming system, and a sheet feeding source designation control method, which are capable of preventing a user from failing to carry out proper settings to thereby avoid printing using an undesired sheet cassette, and also capable of allowing a user to change to another type of sheet feeding than manual feeding immediately if he or she desires to do so, and storage media storing programs for executing the methods. The size of at least one sheet for manual feeding can be designated by a device or a job. It is determined whether or not a manual feed tray can be designated as a sheet feeding source if the sheet size is designated by the device or the job, and the manual feed tray is designated as the sheet feeding source when it is so determined. A sheet selecting screen is displayed on a screen for allowing a user to confirm or change the designation of the manual feed tray as the sheet feeding source upon completion of the designation, if the size is designated by the device or the job.