This is defined in BIP141, section "Block size":
Blocks are currently limited to 1,000,000 bytes (1MB) total size. We
change this restriction as follows:
Block weight is defined as Base size * 3 + Total size. (rationale)
Base size is the block size in bytes with the original transaction
serialization without any witness-related data, as seen by a
Total size is the block size in bytes with transactions serialized as
described in BIP144, including base data and witness data.
From this it follows that
witness are all considered witness data for weight (and therefore fee) calculation, because these are exactly the fields not present in the original serialization..