Video tutorial by Ralph Grabowski
When you double-click an entity in a BricsCAD drawing, the Properties bar appears -- most of the time.
Double-clicking a hatch pattern, however, brings up the Edit Hatch dialog box; double-clicking an image brings up the Image Adjust dialog box. All this is controlled by the Customize command's Double-Click section.
This Bricsys TV video tutorial (4:55) describes how to assign commands to double-click actions:
To learn all of the ways to customize BricsCAD, purchase Ralph Grabowski's Customizing BricsCAD ebook from http://www.upfrontezine.com/cb8 ($40). Available for the current release V15, and all releases going right back to V8.