Do manual do VirtualBox (Capítulo 3.4.1):
The default virtual pointing devices for older guests is the traditional PS/2 mouse. If set to USB tablet, VirtualBox reports to the virtual machine that a USB tablet device is present and communicates mouse events to the virtual machine through this device.
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Using the virtual USB tablet has the advantage that movements are reported in absolute coordinates (instead of as relative position changes), which allows VirtualBox to translate mouse events over the VM window into tablet events without having to "capture" the mouse in the guest. This makes using the VM less tedious even if Guest Additions are not installed.