The String value of a StringExpression is obtained by concatenating, in order, the String values of either the operands or the subExpressions, depending on which is given. Source Unified Modeling Language 2.5.1
The substrings are given as either a list of LiteralString operands or as a list of StringExpression subExpressions (but it is not allowed to mix the two). Source Unified Modeling Language 2.5.1
