Project Reactor. Mono.map() vs Mono.flatMap()

Question

What is the principal difference between these in terms of Mono? From the documentation, I read that flatMap acts asynchronous and map synchronous. But that doesn't really make sense for me b/c Mono is all about parallelism and that point isn't understandable. Can someone rephrase it in a more understandable way?

Then in the documentation for flatMap stated (https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Mono.html#flatMap-java.util.function.Function-):

Transform the item emitted by this Mono asynchronously, returning the 
value emitted by another Mono (possibly changing the value type).

Which another Mono is meant there?

Answer 1

Mono#flatMap takes a Function that transforms a value into another Mono. That Mono could represent some asynchronous processing, like an HTTP request.

On the other hand, Mono#map takes a Function that transforms a value of type T into another value, of type R. That transformation is thus done imperatively and synchronously (eg. transforming a String into an URL instance).

The other subtlety with flatMap is that the operator subscribes to the generated Mono, unlike what would happen if you passed the same Function to map.