A variable can be defined using
variable = expression [,] [unit], ...
unit is an optional string, which can be used to associate a unit to a variable. A variable definition is recognized by the equals sign in the first field of the input card. For instance,
R=1 ANG, THETA=100 DEGREE
defines the variables R and THETA, but
THRESH,ENERGY=1.d-8,GRADIENT=1.d-5
does not define variables; here ENERGY and GRADIENT are keywords to be recognized by the program.