This means that the original Murashige-Skoog plant mineral nutrient + vitamins medium was somehow modified, and, eventually, some PGRS were added to it
Well, a legendary article by Murashige & Skoog in Physiologia Plantarum 1962 is entitled: "A REVISED medium for rapid growth and bioassays with tobacco tissue cultures". So, i would address you to the article where your read this phrase and in the following text there must stand an information if and how the original MS medium has been modified. It could be supplemented with some PGRs (Plant Growth Regulators), or simply with succrose, agar, inositole...