When true, causes the Lisp heap and the executable or dynamic library to be saved in two separate files.
If split is nil (the default), then the saved image is written as a single file containing the Lisp heap. If split is t, then the saved Lisp heap is split into a separate file, named by adding .lwheap to the name (as specified by the argument file). When the executable or dynamic library runs, it reloads the Lisp heap from the .lwheap file automatically.
In addition, when saving LispWorks as an application bundle on the Macintosh (for example by using create-macos-application-bundle), split can be the symbol :resources. This places the Lisp heap file in the Resources directory of the bundle, rather than in the Contents/MacOS directory alongside the executable, which allows the heap to be included in the signature of the bundle.
The main use of split is to allow third-party code signing to be applied to the executable or dynamic library, which is often not possible when saving an image with the Lisp heap included in a single file.
LispWorks Delivery User Guide - 15 Feb 2015