What does SYSTEM mean in `<!DOCTYPE people SYSTEM "peopleDTD.dtd">`?

What does SYSTEM refer to or mean in <!DOCTYPE people SYSTEM "peopleDTD.dtd">? It just seems to be standard usage in XML docs and DTD's, but don't know what it actually means开发者_如何学Go. Thanks.

The SYSTEM keyword indicates that the SGML processor should locate the DTD in a system specific fashion. This typically (but not always) means the DTD will be provided as a filename.

— FreeBSD Documentation Project Primer for New Contributors - SGML Primer