When opening up a Revit model you all of the sudden get a pop up message stating that your file was modified by a third-party updater called AREXRevitStart : REXExtension: DReinfWall.
This message is popping up because someone working on the model has the Reinforcement extensions from subscription center installed. This message can be found in the help menu. menu http://wikihelp.autodesk.com/Revit/enu/2014/Help/3665-Developers/0135-Advanced135/0152-Dynamic_152/0156-Exposure156
You can either move forward selecting "Do not warn and continue working with the file" or review this post on how to change the code of the updater so the message stops popping up. http://adndevblog.typepad.com/aec/2012/05/avoid-the-missing-third-party-updater-dialog.html
For a more in-depth discussion here is a great explanation from the Autodesk forums; http://forums.autodesk.com/t5/Revit-Architecture/Third-Party-Updater-Error-Message/td-p/3564050