At the same time that a pond becomes a lake.
Maybe the musical role has some bearing? Piccolos are often used for flourishes (runs, trills, arpeggios), whereas the flute is sometimes given a melodic passage in focus.
Sorry, otherwise I’m no help.
Well in the modern orchestra, the piccolo is an octave higher than the standard Boehm C flute. That’s how composers score for it. An alto flute is a fourth below. Bass flute is an octave below. There are also D flat piccolos.
Fluters interested in band flutes would call High d through high f a piccolo
C , B and Bb would be treble , melody, or band flutes
F would be F flutes
But I wouldn’t trust them they call the low Bb and the Eb a bass flute.
Similarly Thomas Aebi calls his G flute (made to play with Basel d piccolos) a bass piccolo (not joking here he really does).
An even older nomenclature was naming flutes for their distance from D
So an F flute is a third or tierce flute
G a 4th
High d and 8th or ottavino
It’s all so unclear and terminology changes over time.