Iron sights can be a bit confusing. This page will consolidate most of the necessary information to make an educated purchase to setup your slide for backup sights.
In the past we have separated iron sights by "lower third co-witness" and "absolute co-witness", this is confusing because it does not work the same as a rifle optic where this verbiage comes from.
We now only refer to them as back up sights and generally you want to run the lowest possible set to utilize the most of your optic's window while still being usable should the dot go down somehow and you need to switch to the irons.
Another detail when choosing irons that folks can get wrong, is that your iron sights are a separate sighting system than your dot. You should not marry them. Zero your dot to the distance you want to zero it, with the ammo you want to use. Then group other ammo you might use to know how it differs. Then you should know your point of aim and point of impact with the irons. Dots are far more reliable than they used to be, so chances are you won't ever really NEED your irons, so you just need to know where it's going to hit should you need them.
This information will be separated by slide make and then by optic/optic cut.