While reading latest IG codex i seen this:

"sometimes, two very different regiments are combined with unexpected benefits, as happened with the 182nd Catachan and 90th Elysian, the combined regiment named as the 314th Prosan"

So i was wondering, are mixed regiment named after planet or something? If we combine for example Cadian 90th and Tallarn 112nd how they should be named?

My understanding is that regiments are named after the world they're formed on. The 314th Prosan was named that way because the two units were combined on the planet of Prosan.

So, whatever planet you were on when the 90th and 112th were reformed into a new regiment, would be their new name. And the number would be how many regiments had been formed on that world before, if you wanted to get that detailed.

I think the exception to this rule is that when two units of the same planet are combined, they keep their original name. So if the Cadian 27th and 90th combined, they would be the 27th/90th Cadian, regardless of which world it occurred on.

My understanding is that regiments are named after the world they're formed on. The 314th Prosan was named that way because the two units were combined on the planet of Prosan.

So, whatever planet you were on when the 90th and 112th were reformed into a new regiment, would be their new name. And the number would be how many regiments had been formed on that world before, if you wanted to get that detailed.

That makes sense, I think I'd do it the same way.

Not sure about the "mixed" name from two regiments of the same world, however - it just looks weird. In the case of Cadians, it'd sort of make sense, as it'd just be a temporary name as the soldiers would be rotated back into Cadia's Interior Guard once the regiment's tour of duty is over, but with all others I'd say the smaller unit would simply be absorbed by the larger one, and adopting it's number and any honorific bynames the regiment may have earned.

The 301st and 296th Valhallan combined to make the 597th Valhallan. The mess of regiments in Armageddon's jungles combined to make the Armageddon Ork Hunters.

A third possibility is for the remnants of one regiment to be absorbed INTO the other, larger, better organized one under its name -- for example the 23rd Bruttiam from the 3rd Edition codex includes squads of the 9th Cardosian and 116th Catachan, and a platoon of the 17th Malcomb (the rest of which was wiped out by Pratt's Scurvy). Also 25 enlisted local scouts from the planet of Xenthorp on which they're all stationed. If one platoon of Catachan Devils is attached to a rookie Cadian regiment to acclimate them to the jungle on a new world, the veterans will be considered part of the Cadian regiment for all intents and purposes -- including supply, which means that once their own equipment runs out they will end up armed with Kantrael lasguns and wearing Cadian armor instead of Catachan lascarbines and fatigues. They might even be renamed "Scout platoon, XXth Cadian" instead of "Ambush platoon, XXth Catachan."