Cuis Slang is a port and re-imagination of Slang. And now what is Slang? Slang is a variation of Smalltalk which is so reduced to be used only for writing lowlevel code. Slang was used in the Squeak Smalltalk System, on which Cuis-Smalltalk is based on for writing parts of the Squeak VM. There Slang can be found in the VMMaker project which contains the Slang code generation and the actual VM implementation. In this original use Slang was used for writing the portable parts of the VM whereas the platform specific parts where located in platform specific C files. The whole configuration of this original use can be found in the https://github.com/OpenSmalltalk/opensmalltalk-vm repository and also the VMMaker repository located at (http://source.squeak.org/VMMaker). Here the wiki documentation regarding this original use: https://wiki.squeak.org/squeak/2105
Cuis Slang in contrast to the VMMaker version of Slang is not intended (yet?) for building a whole VM, but for writing FFI loaded plugins and other more general small programs which should be portable.
Slang is as written above a reduced subset of Smalltalk for lowlevel plumbing, in this regard Slang is useful for representing the same program in Smalltalk, simulating the behaviour there and then translating it to different output languages.
Cuis Slang is thought to be more general, it was adapted specifically to this task of translating objects into programs which follow a specific pattern of programs. So for example cookie cutter plugin code, which should follow a specific form for performance, security or maintainability reasons. The VMMaker slang is specifically tailored for C translation of VM Code.
Also Cuis Slang has a little bit of higher ambitions than VMMaker Slang, it's intended to be translated into different languages and not just C, so all features which were too specific for C were changed to be more in a more general way useable.
Codewise Cuis Slang is based on the VMMaker Slang and the SqueakJS version of Slang, so there are areas which look similiar, but were adapted to Cuis and the original intention of being not just for VMs.