如果解析后的上下文变量内容为空，则不会发生替换，并且变量名称在查询字符串中保持不变(包括“%”符号)。

If the resolved context variable content is null, no replacement occurs and the variable name remains unchanged in the query string (including the "%" signs).

youdao

变量名称作为开始和数据类型为字符串类型。

Name the variable as launch and make the type as string type.

youdao

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。

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.

youdao