Ho wdo you go about limiting a convention if it gets ambitious? Remember, the convention that gave us the Constitution was only charged with amending the Articles of Confederation. Insetad, it threw them out.
But it still had to be ratified by the states.
An Article V Convention is the property of the states, and the language used by the states to request Congress to call a convention defines the purview of that convention. In its petitioning language, the states may ask for a convention to address one subject, a plethora of subjects, or even ask for a general convention to address any subject, i.e. a revision of the Constitution.
The states define what the convention is permitted to address.