The term "90c" refers to a specific patch update (version 9.0c) for Sound Forge Pro 9.0. This update was released by Sony Creative Software (Sony sold the software line to MAGIX in 2016) to fix bugs and improve stability in the version 9 series.

The 9.0c update was a stability and compatibility patch released during the software's active lifecycle. It introduced support for new file formats and resolved various bugs present in the initial 9.0 release.

Why not just use Audacity (free) or Reaper (cheap)? Because Sound Forge 9.0c has unique strengths:

If you are trying to authenticate a legitimate copy of Sony Sound Forge 9.0, you may encounter difficulties because the product is end-of-life.

MAGIX Software (which bought Sound Forge from Sony) offers a grace period for legacy owners.