What it means
It can be based on a ratio, chosen optically or adapted to the content. The useful part is the repeatable relationship between sizes, not obedience to a formula. A website may need fewer steps on a small screen while keeping the same hierarchy of title, section heading, body and metadata.
Use it in practice
Define a small set of roles and test them together with actual copy. Allow long headings to wrap naturally instead of preserving one desktop composition.
Watch out for
Do not add a new size for every exception. A mathematically tidy scale can still feel wrong in the chosen typeface.
