  Building on the comprehensive White Paper on the scientific case for
multi-object spectroscopy on the European ELT, we present the top-level
instrument requirements that are being used in the Phase A design study of the
MOSAIC concept. The assembled cases span the full range of E-ELT science and
generally require either 'high multiplex' or 'high definition' observations to
best exploit the excellent sensitivity and spatial performance of the
telescope. We highlight some of the science studies that are now being used in
trade-off studies to inform the capabilities of MOSAIC and its technical
design.
