If set to t, starts multiprocessing with the delivery function (that is, the first argument to deliver) running in a process created specially for it.
If set to :manual, allows multiprocessing to be started by the delivery function, which should call mp:initialize-multiprocessing.
If set to nil, multiprocessing cannot be used in the delivered application.
The value of this keyword argument is automatically t when :interface is :capi, so you only need to supply it if CAPI is not being used.
LispWorks Delivery User Guide - 10 Aug 2017