Quoted:
Mounted directly on the rail will give you a co-witness, dot on the top of the front post.
If you want the dot above the front post, then you want a 1/3 co-witness and need to use a spacer.
Some eotechs, like the 553 and exps, have a built in spacer. You should read up on what a cowitness is, it's been my experience that a lot of people aren't sure what it means. Basically, a standard height eotech reticle will rest exactly on top of your front sight post when properly sighted in. A raised eotech, whether it is by spacer or just a taller model like mentioned above, will have the reticle on top of the front sight post when viewed through the top of the lower 1/3 section of the eotech window