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.
Likewise rūjēmma is rū NP ēmma without a specified location, and so means ‘out’. The expected expression rājēmma is not used.