A typo I think, should be BM25F. From Wikipedia:

> BM25F (or the BM25 model with Extension to Multiple Weighted Fields) is a modification of BM25 in which the document is considered to be composed from several fields (such as headlines, main text, anchor text) https://en.wikipedia.org/wiki/Okapi_BM25

Some papers are linked in the references

Thanks, really appreciate it!