Which methods are available for NDEigensystem?

I am trying to find out what methods/options are available for NDEigensystem and descriptions of their use. Perusing the help, online Q&As, Mathematica’s in-line autofill, etc., I’ve found a few, but it’s unclear what they do. For example, what is the a difference between Method -> {"PDEDiscretization" -> {"FiniteElement", "MeshOptions" -> {"MaxCellMeasure" -> Automatic}}} and Method -> {"SpatialDiscretization" -> {"FiniteElement", {"MeshOptions" -> {MaxCellMeasure -> Automatic}}}}. To confuse things further, some Methods seem have additional options (e.g., "Eigensystem").

Is there a way to uncover all the available Methods available to NDEigensystem, including a description and their options?