What does AD FILUM AQU mean?

AD FILUM AQU meaning in Law Dictionary

into the bond for the liquid; to your main range, or center of stream. Usque ad fllurn aquas, as far as the bond for the flow. Bract, fol. 20S6; 235a. A phrase of frequent incident in modern legislation; which advertising medium filum aqua: (q. v.) is yet another type.