In an assignment statement, the operation field contains an equal sign (=). See Chapter 5. One or more blanks terminate the operation field. The operand field contains entries that identify data to be acted upon by the operation defined in the operation field, e.g., source or target address for the movement of data, or immediate data for storage or adding to another value.
Some statements do not require use of the operand field. For those that do, the operand field usually consists of one or two expressions (the second expression is separated from the first by a comma.) Expressions can be composed of numbers, string constants, labels and symbols combined with arithmetic, logical and relational operators. the Section called operand expression evaluation describes in detail the types of terms used in expressions, the permitted operators, and the order of evaluation used for expressions with more than one operator.