A symbol naming a foreign block type defined using define-foreign-block-callable-type.
with-local-foreign-block allocates a foreign block using type, function and extra-args in the same way as allocate-foreign-block, but with dynamic extent. It then binds foreign-block-var to the foreign block and executes the code of body.
with-local-foreign-block can be used only if the code in body can be guaranteed not to invoke the block or a copy of it either outside the scope of
with-local-foreign-block or in another thread. Unless you can be sure of that, you need to use with-foreign-block.
with-local-foreign-block can be a little faster than with-foreign-block.
LispWorks Foreign Language Interface User Guide and Reference Manual - 29 Sep 2017