My guess it that it's the valence state of the iron. Standard
rust is Fe2O3 (where iron is +3); another oxide is FeO (where iron
is +2), and I believe there's another that's Fe3O4, in which iron
is partly +2 and partly +3 ( a non-stoichiometric compound as I
recall from geochem class years ago). There are more oxides or
iron that are also hydrated - iron's one of those changelings that
really gets around, being different things every time.
Mill scale is ferric ferrous oxide (Fe3O4 or FeO-Fe2O3) and rust is a
poorly understood mixture of hydrated ferric oxide (Fe2O3-XH2O) and
ferric oxyhydroxide [FeO(OH)]. (I am having to use a - where a dot
would normally be.) Mill scale (called fire scale by the blacksmith)
is produced by surface oxidation of hot worked iron or mild steel.
Rust is produced by a poorly understood chemical process involving
iron, water, oxygen and carbon dioxide. It is ironic that in an age
when rocks on another planet are being analyzed we do not completely
understand the nature of rust or the process that produces it.
Ferrous oxide (FeO) is the fourth oxide of iron. The powdered ferrous
oxide of commerce is easily ignited and burns rapidly. This is a
characteristic that has caused explosions.
"Marlinespike Seamanship in Precious Metals"