Variable type ('C' for continuous, 'B' for binary, 'I' for integer, 'S' for semi-continuous, or 'N' for semi-integer).
Binary variables must be either 0 or 1. Integer variables can take any integer value between the specified lower and
upper bounds. Semi-continuous variables can take any value between the specified lower and upper bounds, or a value of
zero. Semi-integer variables can take any integer value between the specified lower and upper bounds, or a value of
zero.

Refer to this section for more information on variable types.