What are set of rules I would use to figure out if bilverdin-IX alpha is an 15Ea,15Es,15Zs or 15Za isomer, likewise for C4=C5 & C10=C11 ethenes?

I have done extensive research on web on this problem and read countless articles, where everybody loves to use that terminoloy in their publications, but no one can explain how. Are there any porphyrin organic chemists out there?

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