Smith's Dictionary of Greek and Roman Antiquities, John Murray, London, 1875 (via LacusCurtius) has the answer: "The word Rogatio (from the verb rogo) properly means any measure proposed to the legislative body, and therefore is equally applicable to a proposed Lex and a proposed Plebiscitum …"
The above can be found in the entry Lex:
Lex is defined by Papinian
(Dig. 1 tit. 3 s1):— "Lex
est commune praeceptum, virorum prudentium consultum, delictorum, quae
sponte vel ignorantia contrahuntur, coercitio, communis reipublicae
(de Leg. I.6)
defines it thus:— "Quae scripto sancit quod vult, aut jubendo, aut vetando" (see also de Leg. II.16).
A Law is properly a rule or command of the sovereign power in a state,
published in writing, and addressed to and enforced upon the members of
such state; and this is the proper sense of Lex in the Roman writers.
In contrast to Lex there is Jus:
JUS. "All people," says Gaius (Gaius, I.1), "who are governed by Leges and Mores, use partly their own law (jus), partly the law (jus) that is common to all mankind; for the law (jus) which a state establishes for itself is peculiar to such state, and is called Jus Civile, as the peculiar law (jus) of that state. But the law (jus) which natural reason (naturalis ratio) has established among all mankind is equally observed by all people, and is called Jus Gentium, as being that law (jus) which all nations follow. The Roman populus therefore follows partly its own peculiar law (suum proprium jus), partly the common law (commune jus) of all mankind."