IEC61131-3标准中变量前缀%是什么意思？

Null is a term used in computer programming and databases to represent the absence of a value or the lack of any data in a particular field or variable. It is often used to indicate that there is no valid or meaningful data for that field or variable, and can be used to avoid errors and improve program efficiency. In programming languages such as Java or Python, null is a reserved keyword that can be used to represent a null value.