What does (Client certificate not present) mean for Yandex?

Question

Yandex

In all letters sent to Yandex, the headers contain the value (Client certificate not present)
/// I wondered how to defeat this dirty trick!
*** I look at letters from different services and those who have a certificate and not, even Yandex itself has this value in the letter headers.
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Accordingly, as I understand it, this inscription does not promise big problems - but for the sake of searching for the best, I would like to first understand where the toes and toes grow from this line.
// Google did not answer this question - apparently few people bother.
I asked Yandex - Yandex did not answer me, just like in the songs (partisan is silent)