Good question.
I have not seen a breakdown. But it could be the following.
Sama = “same”
atta = “truth” or “refuge” or “Nibbana” depending on the context
niyama = principle or law
When you put all that together it rhymes like “Sammattaniyāma.”
So, when someone gets to Sammattaniyāma one “starts taking refuge in Nibbana.”