Rule of Commutation

Disjunction
Its abbreviation in a tableau proof is $\text{Comm}$.

Also known as
The rule of commutation is also known as the commutative law.

Note that this term is also used throughout mathematics in the context of addition and multiplication of numbers:
 * the Commutative Law of Addition
 * the Commutative Law of Multiplication

so it is wise to be aware of the context.

Also see

 * Rule of Association
 * Principle of Commutation