I have created my modules position in my template and have also declared in the XML file. Now in module manager when I choose one of my module positions and save the module, it shows it then as a Custom Position in the Module Positions select field.
Update:
I am not using any framework. Just casual joomla template. Module positions are defined in the XML and in the template's block files (so there should not be considered as custom positions) and are displayed in the Module Position filter in Module Manager normally.
But after saving a module, then it will add it also under the Custom Positions group in the filter, ending with the same module position exist twice, 1 under the mytemplate positions and 1 under custom positions.
It does make things a bit confusing.
Update 2
I just realized that this is also happening by default even with the protostar template, and probably with other templates as well. After assigning a module to position, then this position becomes available also in the Custom Module Positions. Isn't this the case also for anyone else?