Are hallucinations mostly 'real' fantasies or are they 'real' memories? When someone has a hallucination of a person or odor does is that a fantasy which one have never seen before (so a kind of construction) or can it be a remembering but then more real as seen in the mind? 

Are they considered to be the same or is there made a distinction in therapy (if necassary)?
Hallucinations involving a person or odour can be either from memory or completely made up (constructed) within the mind.  Where they are from memory they generally, but not always, take the form of flashbacks which are very intrusive in the sense that they are unwanted, and can be triggered by something the person sees or hears, but again, not always.

Neuroses and psychoses which involve flashbacks can take on any form in any combination of any 1 or more of the senses of the body:


Sight (Visual flashbacks)
Sound (Auditory flashbacks)
Smell (Olfactory flashbacks)
Touch (Kineasthetic flashbacks - sometimes referred to as tactile flashbacks)
Taste (Gustatory flashacks)


Psychoses such as Schizophrenia can involve the same hallucinations but they are different to flashbacks as they are not necessarily recalling an event from the past.  They can be completely constructed.

When a psychiatrist is making a diagnosis, using their knowledge of ,or through reference to, the DSM, they will need to be aware of the form and source of the hallucinations which will play a part in how they will form the diagnosis and treatment plan through medication and/or therapy.  The latest edition of the DSM is the Fifth Edition (DSM-5)

Therapists will need to be aware of how the hallucinations form with their client in order for the therapist to effectively help their clients to deal with them.