Why did oracle start adding letters to their version numbers
Are you referring to how it's 9i (the "i") and 10g (the "g")? If so then it's simply marketing. The letter has no significant importance.
they have meaning, but the meaning is indeed mainly for marketing purposes. When they introduced 10g, "grid computing" was "hot". Now it's "cloud computing" so it's 12c.
And i? Probably stands for "improved" or something like that, adding letters like that to your version numbers was popular (and still is. Think what Apple does...).
If you mean 11gR1 and 112gR2, that's just Oracle's equivalent of "11.0" and "11.5".
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