You can use a field at any location as the tag for the structure of the bitstream that follows that location.
So if you have something like
FieldA/FieldB/tag/FieldC/FieldD
and you need to use tag to determine how to parse FieldA or FieldB, you're out of luck. But if FieldA and FieldB are relatively constant, and you only need tag to determine how to parse FieldC and FieldD, then you're fine.
If you need to use tag to model FieldA and FieldB, you have to use regular expressions, not tags.
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