Question: *Plesionyms* are synonymous words which have slight differences in meaning.

What are the examples of it? I found:

* Fog v Mist
* Fearless v Brave

When and why are they are used?

What are the aspects which differentiate plesionymic synonyms from cognitive synonyms?

Answer: 
Why would you choose one word over another, when the two might be synonyms?

The best explanation was provided by [Isaac Asimov](https://en.wikipedia.org/wiki/Isaac_Asimov):

> 
> R. Daneel said, "I do not understand the distinction you are making,
> Partner Elijah. Since 'murder' and 'homicide' are both used to
> represent the violent ending of the life of a human being, the two
> words must be interchangeable. Where, then, is the distinction?"
> 
> 
> "Of the two words, one screamed out will more effectively chill the
> blood of a human being than the other will, Daneel."
> 
> 
> "Why is that?"
> 
> 
> "Connotations and associations; the subtle effect, not of dictionary
> meaning, but of years of usage; the nature of the sentences and
> conditions and events in which one has experienced the use of one word
> as compared with that of the other."
> 
> 
> "There is nothing of this in my programming," said Daneel [...].
> 
> 
> *(from ["The Robots of Dawn"](https://en.wikipedia.org/wiki/The_Robots_of_Dawn))*
> 
> 
>
When words have a similar but slightly different meaning, there will be contexts where one is more appropriate than another. In other contexts, the difference may not be relevant so either is acceptable.

In your examples: A brave person and a fearless person may both perform the same feats that others may be too fearful to perform. However, a fearless person would not be afraid to perform the feat whereas a brave person may perform the feat despite being afraid. From an outside observer's perspective, if the person did not appear to be afraid, then the observer might use brave or fearless to describe the person. If, however, it was clear that the person was afraid but did it anyway, then only brave would make sense.