Thanks for reading and responding.
The definition you provide from the documentation is the accurate one.
Just and fromCallable is used to analog it for simpler understanding i.e. If using Just, the value is generated before one subscribing, while using fromCallable, the value is only generated when one subscribe to it. Hence such behaviour matches the definition well.
We can use a subject to show the "hotness", but it makes one harder to comprehend unless one already familiar with subject.