An IMPS is linked to initial appointment/enlistment and a ROSO is linked to high value and/or long-term training. Both are an obligation for you to serve for a minimun period of time so the ADF can recover the costs of your training. As an example, pilot training in the Army incurs the same ROSO, however the IMPS will differ based on the level of officer training undertaken prior (SSO, GSO, ADFA, in-service Corps transfer etc).