Put a help link on every page
One link to help on every page, pointing at help for that page. G71 is that technique, and W3C files it as sufficient for its help rule, which is Level AAA. Read that carefully, because it gets confused with the newer rule about consistent help and the two ask for different things. The AAA rule is about offering help at all. The newer Level A rule does not require you to offer help, it requires that whatever help you already offer turns up in the same relative place across your pages. G71 answers the first of those and not the second. A link is also not the only shape help can take. A help link sitting beside a single control, or placed inside a field's own label so a screen-reader user meets it while filling that field, does the same work more precisely.
How we find it in an audit
We look at where help lives and whether every page that needs it can reach it. Forms get the closest attention, since a form is where a person gets stuck with data half entered. A reviewer checks whether the help is specific to the page rather than a generic support index, and whether opening it costs the reader what they have already typed. Help that opens in a new window keeps the form intact, which is why W3C mentions it at all.
How affected users experience it
Getting stuck on a form is worse when you cannot scan the page for a way out. A screen-reader user who does not know what format the reference number takes has to explore the page to find help, then find their way back to the field, then remember where they were in it. A help link inside the field's own label removes all three steps, because it arrives at the moment the question comes up rather than somewhere the reader has to go looking.
Passes vs. fails
Passes
Every page header ends with Help, landing on contact options and answers, and each tricky field carries its own help link in its label.
Fails
Support is reachable only from the homepage footer, and the checkout and the forms offer no route to a human at all.
Other ways to satisfy this rule
6 guides on this site are filed under 3.3.5 Help. W3C lists this one as sufficient for that rule on its own. Implement it correctly, in a way your readers' software actually supports, and the rule is met.
This guide is our interpretation of W3C technique G71: Providing a help link on every web page. W3C publishes its techniques as guidance rather than as the standard, and says so on every one of them. The success criterion is what conformance is measured against, and a technique is one documented way to meet it.