Spacing. That is the problem with Boots, Racks, and ATC Spindles. 

If choosing a Z-Independent Boot (V3 / V9) (stays in position), you need to provide enough spacing between the bottom of the boot and the top of the tool holder. (Flush Rack)

If choosing a Z-Dependent Boot (V5 / V7 / V10) (rises and falls with the spindle), you will need to have enough spacing around the boot to perform the tool change. This means both left to right, as well as front to back. (Tall / Short, Magnetic / Static Rack)

See Also:

Tool Rack Layout