When you double-click on a DGN file, you open it with the full set of MicroStation Power Platform functionalities. Using these functionalities is only allowed with a sisNET license, whose license costs includes the Power Platform. Using these capabilities is not included in the license costs of sisFLOW or sisVIEW and is therefore not allowed with these products.
Note: It takes 2 minutes before a sisNET ProductID gets assigned if you double-click a DGN file. This to allow the sisNET software to make sure what application is being used (sisNET, sisVIEW, sisFLOW, …). After these 2 minutes, the assigned ProductID gets logged to the Bentley licensing server. No worries about the first 2 minutes, these don't get counted as usage.
You have the different options to set the right license usage, depending on which product you want to use when double-clicking a DGN file: