Difference between Mono.then and Mono.flatMap/map

Say I want to call a webservice1 and then call webservice2 if the first was successful.

I can do the following (just indicative psuedo code) :-

Mono.just(reqObj)
.flatMap(r -> callServiceA())
.then(() -> callServiceB())

or

Mono.just(reqObj)
.flatMap(r -> callServiceA())
.flatMap(f -> callServiceB())

What is the difference between the two, when using the mono.just() for a single element?

flatMap should be used for non-blocking operations, or in short anything which returns back Mono, Flux.

map should be used when you want to do the transformation of an object /data in fixed time. The operations which are done synchronously.

For ex:

return Mono.just(Person("name", "age:12"))
    .map { person ->
        EnhancedPerson(person, "id-set", "savedInDb")
    }.flatMap { person ->
        reactiveMongoDb.save(person)
    }

then should be used when you want to ignore element from previous Mono and want the stream to be finised