Using Polygon Mode
The Using Polygon mode is found in the menu under the Select tool in the Edit ribbon group on the Design tab, and under the Select tool in the Edit Group on the Sheet Metal tab. You can also right-click in the design window and choose Select > Using Polygon. This selection mode enables you to select all objects within a polygon you create. Each time you click, a new starting point for the next line is created. You control the length and angle of each polygon line you create.
Use this mode to draw around a specific geometry or model feature you want to select, for example, a protrusion of other geometry you may want to clearly identify. Using a polygon shape for selection can apply when you are, for example, trying to select an irregular or angular shaped part, or assembly of parts.