This keyword controls the percentage of extra bands in addition to the number of occupied bands. These extra bands are necessary for metals or finite temperature insulators.
It is not possible to have both the NBANDS keyword and either the NEXTRA_BANDS or BS_PERC_EXTRA_BANDS keywords present in the same input file.
PERC_EXTRA_BANDS : 60.0
CASTEP keyword glossary
CASTEP parameters keywords