What does that actually mean?
Opus Dei operates under a personal prelature and it appears to be a quasi autonomous arrangement whereby an order or group, with the approval of the Holy Father operates independently of a Diocese and reports direct to Rome. Obviously, within certain parameters set by the CDF.
This is how Wikipedia defines it:
Personal prelature is an institutional structure of the Roman Catholic Church which comprises a prelate, clergy and laity who undertake specific pastoral activities. Personal prelatures, similar to dioceses and military ordinariates, are under the governance of the Vatican's Congregation for Bishops. These three types of ecclesiastical structures are composed of lay people served by their own secular clergy and prelate. Unlike dioceses which cover territories, personal prelatures —like military ordinariates— take charge of persons as regards some objectives regardless of where they live.
Still not a crystal clear definition but at least a blueprint for how the SSPX might be brought back to full communion with Holy Mother Church.
The issue now is, will Bishop Fellay go for it? Undoubtedly there will be some clauses that will cause him to reflect.
And the intransigent three, Bishops de Mallerais, Williamson and de Galarreta, what influence will they have on Bishop Fellay?
I believe that they will not unduly sway the good Superior General who will do what he believes is right, not what others may wish him to do.
Above all (literally) we have the Holy Spirit into whose power we entrust a positive outcome.