Forwards

Key _Forwards in Vol Curve refers to the forward values for the given expiries.
The first value always corresponds to a hypothetical expiry equal to the trade date

Note, this array is generated ONLY IF Sticky Strike = true

Note this key is prefixed by _, which indicates it is regarded as a read-only part of the object's data and therefore its associated value cannot be edited.
For that reason it does not appear as part of the input data.
It carries informational only value for the user and appears as part of its contents.
Also it completely depends on the remaining object contents and therefore is never part of the exported object's data.