SelectionBox
SelectionBox is an object which renders a 3D box around its Adornee when it is a descendant of the Workspace or anywhere where GUI objects are rendered. The box's geometry consists of rectangular prisms forming an outline/wireframe in addition to a surface for each of its faces. By default, only the outline is visible.

† These properties come from this object's superclass, GuiBase3d.
The SelectionBox object does not capture any form of input; it is solely a visual effect. To capture simple pointer input on the adornee, consider using a ClickDetector.
Summary
Properties
Determines the thickness of the boxes outlines, in studs.
Determines the color of the box's surfaces.
Determines the transparency of the box's surfaces, similar to BasePart.Transparency.
Methods
Events
Properties
LineThickness
LineThickness determines the thickness of the box's outlines. It is measured in studs, the same unit for BasePart.Size. If set to 0, the outline will not be visible at all.

Pictured above are three default Parts with default SelectionBoxes applied to them. Their thicknesses from left-to-right are 0.075, 0.15 (default) and 0.3.
See also:
- GuiBase3d.Color, a property of the superclass GuiBase3d which controls the outline's color
- GuiBase3d.Transparency, a property of the superclass GuiBase3d which controls the outline's transparency
SurfaceColor3
SurfaceColor3 determines the color of the SelectionBox's surfaces.
See also:
- SurfaceTransparency, which controls the transparency of the surfaces. You may need to adjust this property in order to see changes to SurfaceColor.
- GuiBase3d.Color, a property of the superclass GuiBase3d which controls the color of the outline rather than the surface faces
SurfaceTransparency
SurfaceTransparency determines the transparency of the SelectionBox's surfaces, similar to the way BasePart.Transparency works. By default, this property is 1, which causes the surfaces to not be visible.
See also:
- SurfaceColor3, which controls the color of the surface.
- GuiBase3d.Transparency, a property of the superclass GuiBase3d which controls the transparency of the outline rather than the surface faces