Because the nozzle of a printer contains an open aperture - it doesn’t “close up” in between extrusions - it is susceptible to an issue known as oozing. In practice, however, the printhead isn’t always capable of holding back material.
The printhead is always either depositing material - to lay down a section of the print - or holding back material in order to move from one area to another. During FDM 3D printing, the printer extrudes filament and deposits molten material in a sequence of shapes.
