All bodies registered with the Homes and Communities Agency must be in the Ombudsman's jurisdiction by effect of the 1996 Housing Act and 2011 Localism Act.
Registered providers of social housing are generally not-for-profit landlords (like most housing associations) but can also be for-profit organisations, as well as local housing authorities with stock (whether retained or managed by a third party, for example an Arms-Length Management Organisation).
Organisations which have been at any time registered with the regulator but subsequently de-registered remain in the jurisdiction of the Ombudsman.
Mandatory membership covers all the housing activities of the bodies concerned as long as they are about the landlord and tenant relation, and all types of tenure (periodic and long lease).
If you have any queries about membership or would like any more information then please email email@example.com.