ēmma
The modifier ēmma refers to the outside of something. So, rū NP ēmma is from the outside of NP or out of NP. The expected expression rā NP ēmma is not used. The word ēmma is related to yesterday’s word mē.
rūjēmma
Likewise rūjēmma is rū NP ēmma without a specified location, and so means ‘out’. The expected expression rājēmma is not used.
I wonder if young children say rājēmma? 🙂
Oh, good question! I would think that they do. It’d certainly be a beginning language learner mistake.