Neighbourhood Forum's and Neighbourhood plan designation - Public forum - Planning Advisory Service (PAS)

I understand that neighbourhood development plans could encroach into part of another parished area providing it is appropriate and there is an agreement between the Parish Council's. However can the same be applied to a neighbourhood forum who's proposed plan area encroaches into part of a parished area? Would this require sign off from the Parish Council if the forum's plan area was deemed appropriate by the LPA? Or would this not be appropriate as a neighbourhood forum can only be set up in a non parished area? Thanks Raj