Mental reference (العهد الذهني) brings a suitable or familiar instance of a kind to mind, without necessarily identifying that same instance uniquely in the external world. The commentary accordingly likens this use to an indefinite expression in meaning; grammatically the noun remains definite through al-.
اُدْخُلِ السُّوقَ
Enter the market.
If a town has several markets and no specific one has been established, the command can evoke an appropriate market as an instance of a familiar kind. It does not demand entering every market in the world, nor must both participants first identify one uniquely determined building.