This is the build number (minor version) of BYOND being run by this client. Typically this is not useful information, but it can come in handy when diagnosing issues reported by players using a beta build.
Clients running versions of BYOND prior to 512 (major version) will not have this information. It is also not guaranteed to exist for non-Dream Seeker connections. When not available, byond_build is 0.