Expression that evaluates either a string containing the name of an environment variable, or an integer corresponding to the numeric order of an environment string in the environment-string table.
表达式，计算为包含环境变量名称的字符串，或对应于环境字符串表中环境字符串的数字顺序的整数。
2
The collection of session variables is indexed by the name of the variable or by an integer index.