The Act of Union continues from post (20)...
"...and that the King's Justices of his Bench, or of his Common Bench of Westminster, shall have full power and authority to direct all manner process to the sheriff and all others officers of the said county of Monmouth, and also to direct writs of Venire facias to the same sheriff for the trial of every issue joined before them; and also to award commissions of Nisi prius into the said county of Monmouth for the trial of such issues joined before them in like manner and form as they do into every shire of this realm of England;..."
A "writ" was a written order constituting a symbol of the power and authority of the issuer, in this case the monarch of England. It required the Great Seal to be applied to become a legal document.
Venire facias = truth:facts = court of inquiry. [My translation, any other input? Please post.]
Nisi prius = unless:except - earlier:former = A earlier case already begun before the act came into force. [My interpretation, any other more precise information out there? Please post.]