These terms are clearly defined in the SFD Glossary.  There is no difference in the way that the SFD process models the performance of decentralised and centralised sewers.  This distinction is included to facilitate discussion in cites/towns where there is both a centralised sewer network and one or more small scale or pilot decentralised systems which may have distinct performance characteristics.  In that case, different percentages can be assigned to variables, for example the percentage delivered to treatment and percentage treated in each case.