What the four letters mean
Expertise – how much users trust the author, website, and business. Usefulness – whether the material helps solve a problem.
Originality – whether it has its own value or is a retelling of someone else's work. Substantiveness – how fully the topic is covered.
Relevance stands alongside: matching not the query phrasing, but the person's actual task.
What this means in practice
An article can be expert and original, but lose if a person was looking for instructions and got a review. Text volume solves nothing here: five thousand characters of fluff are worse across all four criteria than one and a half thousand with specifics and numbers.
How it differs from E-E-A-T
The idea is the same, but this is a Yandex framework, not Google's, with its own wording. There is a lot of useful overlap: both require a verifiable author and real experience behind the text. Under EPOS, Yandex specifically stipulates that AI models can be an author's assistant, but must not replace them.