411 REFERENCE · Wikibooks
Blender 3D: HotKeys/Key/P
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Pressing P, when in edit mode separates the vertices you have selected to the object mode. For instance, you create a box in the Edit Mode, and you wish to separate a face from it in the Scene Collection, so you press P to have the face of the box and body of the box become two separate things in 2 different layers.