Selection and Deselection
In LoongBones, left mouse click is for selection. Right mouse click is for deselection.
Box Selection
In the empty area of the stage (areas without bones and images), press and hold the left mouse button, then drag the mouse to create a selection box. When you release the mouse, everything within this box will be selected.
Multiple Selection
Hold the Ctrl key on the keyboard, then click to select multiple items.