Definitions for "Manual flash"
Flash output is controlled manually in manual flash mode unlike in auto flash mode, where flash output power varies automatically according to the selected aperture. Some Speedlights, example like the Nikon SB 27, SB 26, SB-25. SB-24 and SB 20, provide selectable manual outputs (full, 1/2, l/4, 1/8, l/16 etc.), while others provide full manual output only.
Mode Full light output with the given guide number.
The flash output is controlled manually by the user regardless of selected aperture.