I'd always assumed it was derived from a Carto-Imperialis code (although that's as good as saying "it's random". There is apparently a system behind Carto-Imperialis codings, in-universe, but it's not anything that's been published or leaked in real life) for either the Rubicon II system, or the Furibundus system, or the Koronus Passage itself, or even the general region.
Of course, it could be derived from an acronym (which opens up more room for speculation), or a corruption/abbreviation of aestimare (used by the administratum for certain tithe calculations; no idea what it means exactly).
Do you have other examples of this Carto-Imperialis code? I've not seen it before.
As for an acronym, I find it unlikely. It's not presented in all capitals and acronyms seem pretty uncommon in Imperial works that I've seen.
I looked up 'aestimare' and it seems to be Latin for "to appraise/assess" or something like that. Not sure how that makes any more sense…
I like the brainstorming, keep 'em coming!
It's actually quite common for an acronym that's been around for a while to lose its capital letters, or to only retain the initial capital (and become a proper noun). Laser, for example, began as an acronym for Light Amplification by Stimulated Emission of Radiation. In a 40k example, the Rhino APC was originally known as the RH1N0 (which was the design code for that STC template. It was originally designed as an agricultural vehicle, IIRC).
Offhand, the only other Carto-Imperialis codes I can think of are 5213X, which is the coding for Ghül; and 56-Izar. I could probably find others on a dedicated fluff trawl, but…