AGIWiki/String
The string data type is used to hold small amounts of text that may change throughout the course of a game. This makes them different from messages; messages cannot be altered by AGI; they are read-only (except for use of format codes as described above) from the logic where they are stored. Each string in AGI can be up to 40 character long (including the null terminator). Strings are represented in source code by the letter 's' and their index (i.e. s0, s1, etc.) The first string, s0, is special in that it is reserved for the player input prompt.
String values can be changed directly by the player through keyboard input, or by converting messages or words typed by the player.
There is still some discussion as to the number of strings available in AGI. According to the AGI Specifications file, some interpreter versions (2.089, 2.411, 3.002.107 and 3.002.149) appeared to have room for only 12 strings, but most appear to have room for 24. It is still not clear if interpreters with room for 24 strings actually support 24 or not.