You're talking about a GM specification for a type of hose. Whether it's marked or not doesn't really matter. The industry says hey, that's a non-reinforced emissions hose made of ECH polymers, usually good for extreme temperatures and excellent fuel and oil resistance. It can come in any size or configuration. GM obviously marked them for identification during manufacturing, but nearly all good brand emissions hoses meet that criteria. The manufacturer's specification sheets for the hose can tell you what specs it meets.
Here's some 6' sections of vacuum hose from Highway Stars that duplicates the markings, only they're 5/32". I didn't see any 5/16" though.