According to the documentation this is an illegal combination, but experimentally it is used by Apple. It seems to be used when the argument should be a pointer to a (Clang) block, which is the foreign type
fli:foreign-block-pointer in LispWorks. Since this is not documented, it cannot be relied on.
LispWorks Objective-C and Cocoa Interface User Guide and Reference Manual - 15 Feb 2015