Randomize mode can be used to add some randomness to any kind of selection, which can be helpful when you are creating natural (caves) or distorted (CAVES IN HELL!!!) environments.
Menu path: Mode -> Randomize.
Action category: Transform.
Default key: Ctrl-J.
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This function can be applied to any kind of selection from both Classic and Visual modes.
Depending on selection type, different sets of settings can be used:
When vertices or linedefs are selected in Vertices or Linedefs mode, or only walls are selected in Visual mode, you'll get this:
When sectors are selected in Sectors mode, or floors/ceilings are selected in Visual mode, you'll get this:
When things are selected in Things or Visual mode, you'll get this:
You can use 
button to update random values for a given property.