When opening a map from a WAD file, this is the dialog that lets you choose what level to open and what options to use. At the top you can choose the Game Configuration that you wish to use when editing this map. When you select the game configuration you need, Doom Builder will show you a list of maps from the WAD file that can be opened with the selected configuration. You must select a map from the list to open.

Resources

The Resources area allows you to specify which resources are loaded when editing this map. The Strictly load patches between P_START and P_END only for this file checkbox applies to the wad file you are editing (where your map is in). When this is checked, Doom Builder will resolve texture patch conflicts by strictly loading patch lumps only from between the P_START and P_END lumps as it was intended by id Software, but most sourceports do not require this (as result, older maps have texture patch lumps that are not in between P_START and P_END).

In the list of resources you can see all the resources that will be loaded. Doom Builder shows the items from the game configuration in gray at the top of the list, because those are always loaded first. You can change the resources loaded according to the game configuration in the Game Configuration Window. You can drag the resource items (except the ones from the game configuration) to change their order. See also the Resource Options Window.