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Once they reach series Z does it go back to A or do we get a new format like AA, AB ?

We live in Unicode times. We switch to Greek alphabets.

α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ σ τ υ φ χ ψ ω

After those are used up, it moves to Devanagari, Hangul, Katakana, Hiragana, and then Kanji.