A name or string attribute refers to an object, or a string.

A logical attribute selects or deselects an option.

An integer attribute is a counter, as for repetition in a beam line.

A real attribute defines a value stored as double precisiom data.

A real expression defines a datum for a command, it may be varied in matching. An expression is built of a combination of operator and operand.

A constraint, specifies a matching constraint.

A variable name selects a variable to be matched.