Fragrance

Many creatures, here and in other worlds, communicate through smell. In the case of Witnesses, there is no code or timing involved, not that we know, at least. Their slightly sweet aroma, barely noticeable for humans, communicates full ideas and even detailed descriptions of events they remember with precision.
They don’t produce a different chemical or change patterns to tell those things, they share a memory as if they were in each other’s minds.
How do we know that?
Because they don’t only share thoughts and memories with their own kind, they can communicate with any other creatures, even with ghosts and other Shards manifestations that don’t appear to be living beings. When we finally asked them about their language, the witnesses answered.

For your nose only

By general rule, we can understand their thoughts about us but not the retelling of what they have witnessed. Yet, when asked, the witnesses had confirmed that they do share all those things to their peers, who wouldn’t share it with other creatures either.
They are all discrete together, I suppose.
It's their decision what determines who understand the memories they release into the environment.

We only witness and remember. We don’t gossip. One day, when it’s all history, perhaps we will tell. Right now, this message is not for you.

— Nelouise, a witness that happens to be the head biologists roommate.

And that is usually the people who gave origin to that memory or thought, whether it is a comment about something they did, the answer to a question or greeting, or even a vivid memory of something they did a while ago.
We don't know of any creature that has received a message unrelated to them, and the only ones that have received memories from events in which they didn't took part, are the recipients of letters.

"Spoken" by

witnesses

Understood by

to whom it may concern

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The language of these flowers may be related to telepathy

I had been talking to that thing for years, thinking that he could understand me because he would carry messages for me. It turned out that they don’t know our language, even if they remember every word.
One day, the kind of day when all the bad things happen, I had to hold a knife between my teeth... or was it the screwdriver? it doesn't matter, I couldn't talk when I received a message that I really needed to answer. I nodded, because the answer was yes, and thought "But you don't know that you should tell him that, right Nels?" And there it was, a perfect affirmation in the back of my head, and he was gone to deliver the message.
He had never answered any of my spoken questions, you know? He didn't know what I was saying, because apparently, he can only understand the things I don't say aloud.

— Guierald, Head biologist

Probably due to the origin of the shards that form them, these creatures can understand any unspoken message directed to them, no matter the means and language, yet they are unable to discern the meaning of any complex language.
The do know how to interpret instinctive and unconscious gestures, which is evident in the way memories about those things always are easy to interpret by the recipient of messages even if they aren't capable to understand those things in other circumstances.
While some biologists (and every witness questioned on the topic) consider this interpretations like a part of their thought process, others are convinced that it is part of their communication system and tend to catalog it as a form of telepathy.
Name it as you may, this form of perception is a vital part of the Witnesses' communication, and they had tried—several times, in different ways—to explain how it takes part of the system that allows them to know to whom they can communicate each message.