What is Actor Sequitur Forum Rei?

Legal Definition
According as ret is intended as the genitive of res, a thing or reus, a defendant, this phrase means: The plaintiff follows the forum of the property in suit or the forum of the defendant's residence. Branch, Max. 4.
-- Black's Law Dictionary
Legal Definition
The plaintiff follows the forum of the thing in controversy. See 32 L. R. A. 236.
-- Ballentine's Law Dictionary