Does the plate placed under the sign in this situation indicate the distance from the place where you are obliged to stop?

In the photo, in front of the toll gates, there is sign B-32 with the words "Pobór opłat" (toll collection), and under it a plate "100 m". Sign B-32 means an obligation to stop, and plate T-21 gives the distance from the sign to the place where this obligation starts to apply - here to the gates 100 m further on. The answer is: yes.

The Regulation on road signs and signals says that B-32 "stop - customs control" means an obligation to stop, and instead of the words "Cło-Zoll" (customs) it can have words such as "Pobór opłat" (toll collection), "Kontrola drogowa" (road check) or "Rogatka uszkodzona" (damaged barrier). Plate T-21 under a prohibition sign shows the distance from the sign to the place from which or at which the ban applies. So you stop not at the sign, but at the marked place at the gate, and move off when you are allowed to.

Legal basis: § 26 para. 1 and 2 and § 15 para. 5 point 2 of the Regulation on road signs and signals