How are cuneiform glyphs numbered?

Question

In Sumerian (and thus Akkadian, Hittite, etc) cuneiform, there are often several glyphs which have the same pronunciation (as far as we can tell). So the glyphs pronounced /u/ will be transliterated as U/U₁, Ú/U₂, Ù/U₃, U₄, U₅, and so on.

But how are these numbers assigned? Why is 𒌋 U₁ and 𒌑 U₂ rather than the other way around, for instance?

Answer 1

In theory, the signs with the lowest index numbers are the most frequent. In practice, the numbers were assigned when the pronunciation of signs were first identified. For example, after u1, u2, u3 were identified the next /u/ sign to be identified got called u4.