Is there a name for this aspect of written language?

Every alphabetic character and numeral has defining features. Even if extremely stylized, as is often seen in advertising, or departing far from the standard model, as in young children’s handwriting, it can still be interpreted as long as those defining features are present; without them, it can’t.

For example, a capital E and a capital F look a lot alike, but an F requires exactly two horizontal strokes. More, and it’s not an F. But an E can have many more than three: a child may make a ladder of strokes and we still know it’s an E. Also, we can see an E as an E even without the vertical stroke, but it’s indispensable for an F.

Similarly, the numeral 8 can be distorted in all kinds of ways and still be recognizable, but it has to have two lobes.

I don’t know what to call this kind of attribute, this essential form without which a letter can’t be a letter. I see it as comparable to the idea of allophones in linguistics: alternative pronunciations of phonemes in a particular language that don’t serve to differentiate meaning. But this pertains to writing and not speech. Is there a term for it?