The Census and Social Surveys Integrated System (CSSIS) is a complex statistical process exploiting and integrating the information arising from registers and surveys on socio-economic variables. It is designed as a two phases Master Sample (MS) design based on a set of balanced and coordinated sampling surveys. It is planned for supporting the Istat Population Register (PR) in order to increase the amount of provided statistical information and to improve the level of coverage and quality. For an optimal design of the CSSIS for supporting the PR, it is useful to classify the variables included as totally, partially or not replaceable ones. The main scope of the CSSIS is filling the informative gap of the PR for the estimation of target parameters referred to partially replaceable and not replaceable variables on social and economic data. From the first phase sample a set of negatively coordinated samples of households can be selected for the second phase surveys, aimed to provide information on harmonized and specific socio-economic variables currently observed by Labour Force, Living Conditions, EuSilc and Consumer Expenditure surveys.