Map File
A Map file is just like any other Unreal Engine file packages. Being a package they can contain all types of resources; the .unr file extension is merely a convention to denote "this package contains brushes and actors that make a playable map."
Check [Antonio Cordero's website] for a pretty thorough documentation of the [Unreal package format] and a Delphi unit that implements it.
Game | File Extension |
Unreal Unreal Tournament |
.UNR |
UT2003 UT2004 |
.UT2 |
DeusEx | .DX |