This keyword contains a string which specifies the model file from which data will be read to initialize a new calculation. If no file is to be used, this string should be NULL, which means no reuse (a new run).
DEFAULT (case-insensitive) will use the data from the seedname.check file. The same effect can be achieved by providing the REUSE keyword on its own, with no value.
A run cannot be both a REUSE and a CONTINUATION. A REUSE run is a new calculation that uses as much as data as possible from a previous calculation.
REUSE : DEFAULT
CASTEP keyword glossary
CASTEP parameters keywords