BIDS - b0 correction for DWI

Dear all,

We acquired a b0 with a reverse phase encoding direction to be used to correct DWI (topup). I would like to know where to put such images in the BIDS format. Should it go to the dwi folder or fmap folder ?

Thank you in advance

From the spec it looks like it can go in the fmap directory with an additional json file specifying which files it is intended to be used with: