A. An assertion is typically raised when a program gets to a bit of C code that it shouldn't be possible to enter, or has been entered with wrong parameters. An assertion is raised by the developer to let them know of this bug and to prevent the code running any further and thus cause unknown further problems.
With SQL 6.5 assertions are typically only raised with the debug (checked) version of sqlservr.exe. In the normal supplied version the debug code is removed for performance reasons. Checked versions are normally only provided by Microsoft PSS specifically to help track down bugs with a customer.
With SQL 7.0 it is possible to get these errors from the normal compiled executables.
In any event, if you get one of these errors then you need to raise it as a paid fault call with Microsoft PSS via your normal support channel.