Propellerhead Reason Project File
The Propellerhead Reason Project File (or song file) contains the information on a project in Reason (the digital audio workstation software from Reason Studios, formerly Propellerhead Software, not the "Free Minds and Free Markets" magazine). Depending on what sort of audio data is in the project and whether it was created within the program or imported from elsewhere, the projects might be self-contained with all sound data or include external file references so those other files need to be stored and transferred in order for the project to be usable.
The .reason extension seems to be what is currently used, with .rns used in earlier versions. There are some online references to a .rsn extension, but that could be an error; it's unclear whether such a format variant actually exists.
A number of other extensions are also cited online, including .rsndemo (from a demo version of the software?), .rps, .rei (from a Reason Intro variant), .reidemo (demo version of Reason Intro?), .relt (Reason Lite), .reltdemo (Demo Reason Lite), .ree (Reason Essentials), .reedemo (Demo Reason Essentials), .rltd (Reason Limited), .rsb (Reason Adapted). These seem to be from variants or editions of the Reason software, but the exact differences between the formats don't seem to be documented.