In order to understand the meaning of a neume element or significative letter in the St. Gall manuscripts, we must first consider the context. Remember, the scribes were writing down notes as reminders of melodies that were already committed to memory. Slight differences in the notation of otherwise identical chants from place to place suggest that some singers needed extra warnings not to let the pitch drop in certain places, for example, while others could be expected to perform nuanced rhythms without the more explicit graphic indications given in other sources.
To put this concept in a modern perspective, think about the symbol found above the number 3 on American keyboards, which is known variously as the pound sign, hash (or hatch) mark, or the number sign. The symbol has a clear meaning when it precedes a number (e.g. “#5” = “number five”) and a different, yet equally clear, meaning when it follows a number (“5#” = “five pounds”). Because of its similarity to the diesis, the symbol can also be used in musical shorthand, as in “G#” (g-sharp).
Read more about context and St. Gall chant notation.