Rust-based SlackBuilds とは何ぞや。

http://slackbuilds.org/repository/15.0/development/rust16/
> rust16 installs a limited set of Rust stable binaries to /opt/rust16.
> This is to allow Rust-based SlackBuilds to build more recent versions
> than would be possible with system Rust.

なんでそれが、
http://slackbuilds.org/repository/15.0/system/clamav/
において、 This requires: に指定されているのか。