Google product feed identifiers for V2 feed

Question

We are migrating from v1 to v2 for google product feed and following the instructions provided by google https://developers.google.com/product-review-feeds/v1_to_v2_migration/

In v2, google has divided the product unique identifiers to gtins, mpns, skus. In v1, we are using MPN, UPC, EAN, ISBN, base_docid(gbase). From this link https://support.google.com/merchants/answer/160161, it is clearly stated that UPC, EAN and ISBN comes under the category of gtins and mpn to mpns which is fine. For product identifier base_docid, there is nothing mentioned on Google documentation which category will it come and i am unable to find it anywhere.

Thanks in advance!