Why do we need Flags field in ACE?

According to spec:

"The ADS_FLAGTYPE_ENUM enumeration specifies values that can be used to indicate the presence of theObjectType or InheritedObjectType fields in the access-control entry (ACE)"

So, if ObjectType/InheritedObjectType  isn't null GUID, it is present. Why do we need also flags field to know what is present and what is not?

Thanks,

Olga