In this times, i heard of many feudal titles. Nothing against then, its a RP background. Is clear, the noble titles descend from cultural circles of lands, where is established a feudal system. I know not exactly, what system of titles is established in Forgotten realms, but by my mind, the makers not respect historical rules in this.
This topic is only for muse on this, (especially for montezzis, where the titles are used in this time. )
Title "prince" is very honorable, but too inconsistent in times and places. Roman "principe" was synonym for "leader" and "princeps" was part of titles romans caesars. In this case, the "dux" was title for provincial governor with administrative, judgment and military high authority. From this comes titles "duce" od "duke" as souverain rulers in his domains (Burgundia, Bretonia, Normandia). Title "prince" was in this times practically unknown.
Dukes and kings in Europe was named "the princes" in sense independent rulers. Practically, the "duke" was real title and his bearer was too "prince" in generic sense.
In later times, the "prince" was named in two senses. For first, "prince" was independent ruler, particulary in German Empire (fürst, reichsfürst). (But here was too title "kurprinz" for succesor of ruler (only in case the "kurfürst" = "elector count")) For second, prince is member od rulehouse in land ("princes of blood" in french, "prince of wales" in Britain). As independent ruler, the "prince" = the "duke" or the "prince" < the "duke".
EDIT1: the "prince" was at no time named "prince" XY, but XY, "prince of Z, in our case, for example: "Rinaldo Montezzi, prince of Lower Dunwarren"
In modern times, the "princes" are named members of nobility from title count and higher.
Nobility under rulers was in order of highness: - marquis/marquise - count/countess - viscount/viscountess - baron/baroness - knight, chevalier, yeoman (Britain)/lady
For more, see at Wiki pages, where is some little as well.
-sorry for my english, is very simple-