Data Utility License
The Doom Data Utility License was an attempt by id Software to exercise control over the creation and distribution of modifications for games based on the Doom engine through licensing of the DoomBSP utility.
id requested that several prominent community members developing editing software, such as DEU, sign this license as a legal agreement and make anything created by their tools automatically subject to it. It sets limitations on commercial exploitation and distribution, use of trademarks related to Doom, use of data within the IWAD files, requires utilities to avoid functioning with the shareware version of the game, and excludes id Software from any grant of warranty or legal liability arising from use of covered utilities and data.
Apparently the legal enforceability of the license was never determined, however. Numerous companies such as WizardWorks produced compilation CDs like the D!ZONE series distributing data created by utilities affected by the DUL in direct defiance of those terms, and were never subjected to lawsuits as a result. This would suggest that id did not believe they had a case. Instead, out of eventual frustration with this situation, id turned to leveraging the community's editing skills to their own advantage in the form of two projects, Final Doom and the Master Levels for Doom II.
Text of the Doom Data Utility License at The Difference Engine