AMP instructions vs Ammeter help!

Maybe you can school me please FABO !

I'm installing a new aftermarket wiring harness in my 64 Dart the instructions say
( AMP METER INSTRUCTIONS)
If you are using an Amp meter you will need to route the 10 GA Solenoid PWR=Red wire from the fuse panel to the amp meter and then from the other post of the amp meter out to your positive BAT source.

There are indeed 2 posts on the back of my cluster one marker ALT and the opposite one is unmarked! Although I've read that there is a difference between an ammeter and an amp gauge

Also in the diagram it shows 3 posts, one of them a ground!
New to electrical so in English please, ready to fire her uP & don't want to melt her down!

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