This is the tool radius, which is also displayed in the editor view.
If this option is active, only visible guides can be selected. If this option is not active, elements on the reverse side of an object will be selected as well.
If the Hair object is polygonal, the selection of guides can be restricted to a specific polygon selection (Cinema 4D must be in polygon mode).
If this setting is active, locked guides can be selected as well.
Use Shape to define the shape of the tool: Circle, Box or Diamond.
If you use a graphics tablet, use this setting to define which settings will be affected by the pressure of the pen.
Use this setting to stretch the shape of the tool in a horizontal direction.
Use this setting to rotate the shape of the tool.