As nobody with an answer has responded permit me to be the first to admit I don't know. I think perhaps I should, though, and I even may have known at one time or another in the distant past. Fluxes perform their duty in several different ways, and it's possible the type numbers refer to the different chemical activity or doubtfully refer to the melting range of the flux. A welding flux must have a lower melting point than both the metal and the oxides it's supposed to protect the weld from, and either prevent the formation of or carry away oxides. Perhaps a third type affords some cleaning action, as I speculate cast iron containing large amounts of graphite may require... regretably I'm afraid my knowledge on the subject is itself in a state of flux... I can merely speculate as I do not recall answering the question on any examination, and its never been used in conversation in my presence. I doubt a "type" of flux as I speculated previously would necessarily melt at an appropriate temperature to be effective for every given filler alloy, and I cannot see a simple, short answer (even if I knew what I was talking about) as to which would be the one to use for any given base metal.
I suspect the flux numbers you named may be brand-specific, and peculiar to the manufacturer, who may be able to give you the formula or at least the functional differences between them. I was unable to find any reference to them, even various flux labels around my shop don't identify themselves as "type 1" or such.
If you find out, please let me know too.
In what context did you hear the fluxes refered to as such?
Regards,
d
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