Picatinny Rail Types

[Mojamz]

I've noticed two types of Picatinny rails, one is a flat style with evenly spaced grooves and the other has the same evenly spaced grooves but it also has a slot cut at the center which runs length ways down the rail. Other than appearance, what are the differences between these two rails?

Thanks
Mo

[bohoki]

the one with the cutout cant use the promag grip funny since promag makes the one with the cutout