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Flat Bed

Nothing is flat. it’s a matter of how small the deviations from flat are. The smaller these deviations are the more it cost. If you want a really flat bed you can spend thousands of dollars and get a bed that is really flat.

Stuff changes size as it changes temperature. As a bed is heated, from room temperature to 55c, it expands. The volume of the plate goes up more than the perimeter of the plate. The extra volume needs to go someplace. The plate goes out of flat and moves up or down in the center or becomes wavy to allow for the extra size. So, the bed you spent thousands on to get flat is not flat when the temperature changs.

Printers can move the nozzle up/down a little bit as it moves around. They use a mesh of the bed to keep the nozzle a constant distance away as the nozzle moves around.

Automatic bed leveling (ABL) uses a probe of some sort to build the mesh. You can have a gcode to create a mesh and to use a mesh in your print. If that gcode was not put there by the slicer ABL is not used.

After you use ABL the display of the bed will not be flat. It will have the deviations of the bed from flat.

Different printers can accommodate different deviations. If you want to know if your bed is flat enough, do a mesh and look at the deviations it shows. Contact the manufacturer of your printer and find out how large a deviation your printer can handle.