From Traderpedia

Definition: Another term for a trading arcade or prop shop.

A Poprietary Trading Firm, is a firm where the traders trade House Accounts.

It differs distinctly from an Arcade in that an Arcade is a "rentable" situation so far as the traders are concerned. Traders being free to come and go as they wish.

Traders at a Proprietary Trading Firm are much more closely linked with the Firm itself, are likely to have contracts and generrally swing a much bigger book