What's the difference between "Initial Layer Width" and "Initial Layer Flow" in Cura?
Cura option Initial Layer Width will cause lines to be further apart or closer together, based on the value you set with respect to the default. The required filament flow to produce these lines is calculated based on the width of the line and the overlap between lines (and layer height).

The Cura option Initial Layer Flow adjusts flow for the current line width with a multiplier, this means that the distance between lines stays the same. I.e. with this parameter you can overextrude to push more material to the build plate. Note that for a well calibrated machine this is not necessary. My printers use the paper method to determine the initial Z=0 for levelling and never use a wider initial line width or overextrusion of the first layer to get perfect filled mirror finish first layers on glass. However, if (paper) tape is used, the bed may be less flat and overextrusion might be beneficial for better adhesion.

The options can be used together, the multiplier will act upon the calculated flow.