PPR relief

Under periods of absence which are deemed to be full occupation,in the last point there is an always exempt of 3 years for any reason while in some questions this 3 years is exempted while in some there is not. In the notes 4.2 there is something like 'Period(b-d) must be preceded ......................' which I didn't understand but this has sth to do with my question . Please explain this to me in simpler terms.