PLN Application Window
In Windows, each file type has a "default application". File types are determined by the extention of the file name (the text after the last "."). When you double-click on a file in Windows Explorer, Windows invokes the default appliation to open the file.
Dodge Data & Analytics has historically distributed drawing files using the extension ".pln". These were actually TIFF files. They just had a different extension. Dodge Data & Analytics's Dodge View application registered itself as the default application for PLN files. If you are using Dimension in place of Dodge View, you may want to register Dimension as the default application for PLN files.
In the User Preferences you can select the option to do this. But that setting is not easy to discover. To aid the user, each time you open a PLN file in Dimension, Dimension checks to see if it is the default application for PLN files. If not, it displays the PLN File Registration Window.
If you want Dimension to be the default application, simply press the "Yes" button. If not, press the "No" button. But if that's all you do, the PLN File Registration Window will be displayed again the next time you open a PLN file in Dimension. To avoid this, Un-check the box "Display this dialog when opening PLN files".
Note: If you select Dimension as the default application then run Dodge View again, Dodge View will re-register itself as the default application for PLN files. This will remain in effect until the next time you run Dimension.