In this context, nothing.
But context matters.
For one gallon kits, estimated OG/FG values are not provided. Instructions on when to top off to one gallon are not provided. Taking three OG/FG measurements will cost two bottles in a one gallon kit (with a carboy, one goes from maybe 8 to maybe 6 bottles). So there are lots of details that go along with suggesting taking hydrometer measurements with one gallon kits.
It seemed reasonable to suggest that someone who is a proponent of taking these hydrometer measurements (my refractometer works for me) update the post with information on how to do this for one gallon kits.
