Since all strings are arrays, all rules which apply generally to arrays also apply to strings. See Section 15.1 (Array Concepts).
For example, strings can have fill pointers, and strings are also subject to the rules of element type upgrading that apply to arrays.
All functions that operate on strings will operate on subtypes of string as well.
However, the consequences are undefined if a character is inserted into a string for which the element type of the string does not include that character.