When split-file processing is in effect, the first
variables in the matrix data file are the split variables, followed
by ROWTYPE_, the case-identifier
variable or ID, VARNAME_, and the distance variables.
A full set of matrix materials is written for each
split-file group defined by the split variables.
A split variable cannot have the same name as any
other variable written to the matrix data file.
If split-file processing is in effect when a matrix
is written, the same split file must be in effect when that matrix
is read by any procedure.