Neil,

You don't mention what instrument you play.

As far as I can tell, most of these really obscure sounding chords are the invention of 'Jazz' pianists.

Don't get me wrong, their descriptions are technically correct, and may make the originator feel 'clever' or 'superior.'

In practise, they are a waste of time.

I mostly play guitar, and only have 6 notes. On a guitar Em7 is the same as G6 in a different inversion.

Try not to get too involved in theory - play what sounds right to your ears...

Wolf