Assert builds are made specially for debugging purposes. They should render exactly the same output as daily and release builds, but are about 50% slower. Assert builds should display an error message once a problem is detected, before the host application crashes.
These error messages will also be displayed in silent mode - for example on nodes when using distributed rendering, where 3ds Max is running in command line mode. The error message will block rendering, so using assert builds on render farms is recommended only in special, supervised debugging sessions.
Assert builds are installed by copying appropriate files from the archive into 3ds Max installation directories (DLL files into root folder; DLR, DLT into /plugins/ folder). To make sure you have installed the assert build properly, you can go to Render Setup > System > About Corona Renderer / Licensing and see if there is a "WARNING: This is slower ASSERT version." message there, or you can go to Render Setup > Performance > Development / Experimental Stuff and press the "[DEBUG] Do Assert / Error" button to see "Corona debug assert" window in addition to standard Corona Error messages.
"Corona debug assert" window serves as a confirmation that the assert build has been installed properly.
When to use them?
Assert builds should only be used when the user is asked for this after reporting a problem. This usually happens in situations when "standard" bug reporting methods fail (such as showing error messages or sending dump files).
Where to find them?
How to use them?