standard
A standard is a rule argued out in committee, published under a number, and identical for everybody who claims it.
In more detail
One word covers two different things, and the difference decides what a promise is worth. The first is a text. A body such as ISO gathers the trade, argues the matter out over years, and publishes the result under a number anybody may buy and read — ISO 19005, ISO 15930. What it asks for is fixed, public and the same for every firm in the world.
The second is a habit: a way of working that spread because it worked, and that the whole trade now follows. It can be excellent. What it has not got is a text to point at, so two firms can both say they follow it and mean different things by it.
This site keeps the word for the first sort. Where something is common practice and nothing more, it says so in those words.
What it does on a real document
A document claiming a standard is claiming something a machine can check, clause by clause, against a published text. That is what lets an archive take a file from a supplier it has never met, and what lets a printing works run a job without opening it first.
It also makes a refusal something you can act on. When a file is turned away, the clause it fell foul of has a number, and that same clause says what to change.
Where it comes from, and why
The PDF format itself started as one company's own way of doing things and became a standard in 2008, published as ISO 32000. Not one file changed that day. What changed is that the rules stopped belonging to anybody, so anyone at all could write a program that reads them or writes them.