The 5th is usually the first note sacrificed in extended Chords. For example (but this is not always the case) The root and third define the chord and the 7th, 9th etc embellish the chord so the 5th can be omitted. But this isn’t written in stone so you will find inverted chords that leave off the Root but the 3rd being it major or minor always stay unless covered by another instrument.
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To me there is no difference. People use various notations to describe the same chord.
I like to keep it simple: an Em9 implies the presence of the b7, so I leave it out of the chord name. The same for (m)11 and (m)13 chords.
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