objc-at-question-mark
is a foreign type corresponding to the '@?' character pair in the type encoding of a method.
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 Dec 2011