While reading some internal documentation, I unexpectedly stopped at the following word sequence: "as most other software available at /usr/prog ...". I've never heard of the directory, but the context is CentOS and I am rather new to CentOS as well.
Rapid web searching shows that the directory is indeed used by different software (in particular, I saw that python installation sometimes lives there), but I can not see where the directory is documented anyhow for CentOS or other systems. Is there a resource where one can read about it?
Local to your site, you can put things wherever you want, including /usr/prog.
No one has bothered to document this as any kind of convention. Linux FHS, as well as hier man pages from a variety of systems, say nothing about it. Who knows where it came from originally.
On a system following FHS, site local software would be in /opt. Maybe /usr/local.