It seems these might be valid transitions but are not represented in the state diagram:
ACTIVE -> CANCELLED
SCHEDULED -> EXPIRED
POSTPONED -> CANCELLED
does this makes sense?
are there any other missing transitions?
Off topic but related, we might want to improve the state descriptions, small example, for the INITIAL state it would be useful to mention it is a virtual state that is there just to make the state diagram valid (have one initial state), and also to use UML state diagrams (https://en.wikipedia.org/wiki/State_diagram_(UML))
I would like to know others opinions about that ^