It’s the actual raw magnetic field changes encoded in the platter. It matters because lots of floppy setups had “anti copying” setups which played tricks on re

It’s the actual raw magnetic field changes encoded in the platter. It matters because lots of floppy setups had “anti copying” setups which played tricks on readers, or where degradation over time means the peaks and valleys of 1/0 can become less pronounced. Thus getting at the core flux can help with retrieval and fixing