The 2PL ala (also used for 3rd person) is used for politeness and deference for 2SG and 3SG ('you' and 'he/she'). A woman even used it in speaking about a child whom she had interviewed. This usage is probably found in other Ubangian languages, as in Gbaya. However, not everyone adheres to the rule; I have been addressed with mo by a lot of kids.
Source: Samarin 1967: 140