The QA40x application is the software that runs the QA402 and QA403 hardware (also known as the QA40x hardware). It is free to download and use, but it only works the QA402 and QA403 hardware.
The different releases of the software are accessed via the "releases" tile on the right side of this page.
Github uses the "Latest" tag and "Pre-release" tags to differentiate between the more stable "production" releases ("Latest"), and the less stable "new feature" releases ("Pre-release"). The "Latest" production release is located HERE and the most recent "Pre-release" is located HERE
The Wiki contains a Getting Started guide for the QA402 and QA403 hardware, as well as an intro to controlling the QA40x hardware programmatically.
The manual for the QA40x hardware is available from the application. Click on Help->Open User Manual PDF.