You can send and receive four character grid squares (ex: BL10) instead of the usual QTH name (MAU, CA, DX, etc) but ONLY if the mode you use cannot send or receive the required name. FT8, FT4 for example cannot exchange QTH names or abbreviations, but it can exchange grid square. So that is what you must log.
If the other station does not send you their grid square, you should still log the QSO but then you have a choice. You can leave the received QTH blank. That will result in zero points for that QSO. Or you can manually enter the grid if you know it, or the correct QTH abbreviation (MAU, CA, DX, etc) if you know that. If that matches the other station’s QTH correctly then you will get three points for a valid digital QSO. It is allowable to look up a call on QRZ to find the published address for this purpose.
For multiplier credit, if you log the grid square, you must accept our determination of what multiplier that grid qualifies for. For example EM71 could be either GA or AL. We will count it as GA because EM71 has more area and population than the part that is in AL. We do not guarantee this to be accurate. We do the best we can.