If you're lucky, there's a detail schedule attached to the K-1 that tells you what it is. If you're not, it's from one of those partnerships that owns other partnerships, and it's been washed through so many returns that no one knows what it is. When I do get some detail, it's mostly ACE adjustments (not relevant to a 1040) or tentative excess IDCs (maybe relevant to a 1040, but not necessarily giving rise to an actual AMT adjustment, much less an AMT liability).

If your client isn't close to AMT, and doesn't have any credits that are limited by tentative minimum tax, and the Code F amount is relatively small, you can ignore it.

If it's potentially material to your client's return, ask the preparer if they have any additional information, because they are the only one who could know what lurks beneath that code.