S.D.I. is an abbreviation for "Strategic Defense Initiative".
Commodore 64 and Apple IIGS ports of SDI were planed but were never released.
On September 23, 1987, S.D.I. was put on the infamous German index by the BPjS. For more information about what this means and to see a list of games sharing the same fate, take a look here: BPjS/BPjM indexed games.
The manual for the game has a nice section at the end that describes many "wars in space" movies if the reader wants to pursue the genre further. Some of the movies mentioned range from the "Flash Gordon" serials and "Radar Men from the Moon" to the "Star Wars" and "Star Trek" series.