You can use the DSCP values in any way you want on your own network (DSCP values are not honored on the public Internet). You can even make EF be the lowest priority value.
You first classify the traffic, then you mark it using DSCP values. You must configure your routers to do something with those values under congested conditions (by default, DSCP values are ignored). What you configure on your routers is completely up to you. You should really be consistent across your entire network, otherwise you can make a real mess of it.
Using too many values is a rookie mistake. You should not use more than about half a dozen. Once you get to that many, it is probably time to rethink your QoS strategy.