The is a later revision of the +2 model. It was manufactured by Amstrad after they acquired the rights to the Spectrum range. This model had specific differences in its hardware and, consequently, its firmware (BIOS), which is why it requires its own dedicated file to be emulated accurately.
If you provide a few more details—like the programming language involved or the specific goal of the work—I can write the actual code, a formal report, or a step-by-step user guide for you. spec1282azip work
The file uses an advanced algorithm (like LZMA2) not natively supported by the OS. The is a later revision of the +2 model