Null also indicates the default calculation method of the default chart and so on.
Symbol () allows you to read the data of other currency pairs in any chart (strategy test failed) and is also used to "prevent ea from running on the wrong chart". For example, adding smybol==gbpusd in the open condition stipulates that this ea can only be used as gbpusd.
Add: I can't code, just an idea. Specific code writing can be found in the mt4 help file.