A boroughs in the Kingdom of Valyria is defined as any settlement of town size or larger which has incorporated itself with a Town Charter. According to game mechanics , this town charter opens up in game for any settlement that reaches 25 parcels, has 75 inhabitants, and possesses a town hall.  The mayor of a chartered town is then recognized by Valyria as a lord by courtesy, styled Lord Mayor, even though their hereditary title that they pass to heirs remains at the gentry level.
The identification of a settlement as a boroughs is important for the sake of the Valyrian social class structure. Boroughs are the dwelling places of freemen, and specifically that sub-type of freeman which is known in Valyria as a burgess. A burgess is literally the inhabitant of a burg or Boroughs, and the word is cognate in several languages with terms like burgchurl, bourgeois, and burgher. The identification is important because of the distinction between the game’s civilian settlements in the hamlet-village-town-city-capitol scheme, and military settlements in the outpost-garrison-fort-keep-stronghold scheme; the latter are the holdings of barons, who are feudal lords served by coloni called sergeants. “Lords Mayor” are neither feudal lords nor vassals in the sense that barons are, and burgesses are citizens of the town they dwell in as opposed to feudal vassals like coloni.
The town charter that constitutes a boroughs, therefore, is a form of collective fealty contract, where the boroughs as a whole becomes the vassal, as opposed to any individual within that settlement, not even the Lord Mayor, who may himself be a landholder with the feudal title of esquire or even baronet.
- Town entry in Chronicles of Elyria Wiki,