This is essentially the distance along the track allocated for the recording of an area of Flux. That is, in each “cell” there are magnetic particles that together indicate a detectable magnetic polarity of set length. Note that bit cells are not read as bits of data directly, they are used to form the flux transitions used to make up the Encoding. Every bit cell contains either a change of polarity (1) or not (0).
From the perspective of Amiga software, data on magnetic media is read and written as bytes, whereas the hardware itself treats them as bits. Writing through the Amiga disk controller hardware is limited to a data “word” (two bytes), and bit writes are clocked by the hardware clock, to produce either 2us or 4us wide bit “cells” for the MFM (Modified Frequency Modulation) Encoding format.
For this reason, modifying the width of the bit cells written using Amiga hardware is limited. This is one of the reasons why Long Tracks have to be copied using a hardware copier, or by changing the drive speed. An alternate method would be to change the clock speed of the FDC, or the system clock in the Amiga’s case since that is what it is derived from. However many games were mastered more professionally, with programmable duplicator systems - like from Trace. These were capable of writing any cell width desired.
Reading the bit cells can be done by timing, at a very fine resolution, how far the flux transitions are apart and then deriving the intended bit cells using some fairly detailed analysis. This is exactly what our KryoFlux device and associated software does.