So it branched off 3.1, and aims to have full source compatibility.
It's a modern Amiga-ish-near-as-dammit OS for people who don't have money coming out of their ears
Things fragmented a bit due to various Amiga rights being tossed around from company to company and it ended up that AROS is based on 3.1 but they can't give it an Amiga name because a different company owns the Amiga trademark. Yes, it's a mess. IMHO, it's as legitimate an "AmigaOS" as the ones that got to keep the name - it's all out of the same code/trademark/rights blender that has been churning for the past 15 years.