What is The Sim Factory?
The Sim Factory (TSF) was a small software company that used the ISIMotor (the same engine that powers rFactor1) from Image Space Incorporated to create realistic stock car simulations.  This video is an interview with Nick Steelman and Robert Coulter in 2008.  TSF worked with real ARCA and NASCAR race teams to develop simulated tracks and cars suitable for driver training.  Years after it's original release, devoted sim racers continue to use ARCA Sim Racing because it offers unparalleled realism.  ASRX History

ARCA Sim Racing 08 (ARCA08) was released on DVD in early 2008 for $50.  ARCA08 is not considered a modification to rFactor because it uses a stand-alone executable that provides advantages when simulating stock car racing not found in the rFactor release.  The sim is intended for online multiplayer use.  The artificial intelligence (AI) is not a key feature of the sim.  TSF published 24 tracks for use with ARCA08:

ARCA Mountain, Berlin, Charlotte, Chicago, Daytona, Daytona Road Course, Duquoin, Texas, Gateway, Iowa, Kansas, Kentucky, Michigan, Milwaukee, Montreal, Nashville, New Jersey, Pocono, Rockingham, Salem, San Jose, Talladega, Toledo, USA International

TSF Trucks (XPatch) were announced in 2011 as a free add-on to ARCA08. The planned release date was February of 2013, but the trucks never emerged from beta testing.  TSF stated that all purchasers of ARCA08 could install the XPatch.
The XPatch contained an updated executable that removed Stop/Go penalties making it possible to do two-wide restarts and wave-arounds without admin intervention.  The trucks also used newer graphical shaders that significantly improved reflections on the truck body.  The truck bodies were laser scanned and are not symmetrical.

ARCA Sim Racing X (ASRX) was created in the winter of 2014-2015 after updates to the XPatch were abandoned. Community members who had previously provided add-on tracks, the spotter plugin, and Sim File Fetcher worked together to combine ARCA08 and the XPatch. Once the ARCA car and Trucks were successfully merged, they created Cup and Grand National cars. Stock Car Evolution (SCE) donated the car bodies and the complete package was made available using the new ASRX Launcher in January 2015. In 2016, the Cup car was modified to simulate the low-downforce package introduced by NASCAR. In 2017, the Cup car was modified again to keep up with lower-downforce aeronydamics introduced by NASCAR.