UU is an 'advice code' that is the outcome of an attempt to confirm a segment which was not able to be confirmed, but was placed on the waitlist. Advice codes like UU (and others like KK, KL, TK, TN etc) need to be 'acknowledged' in the PNR in order for the relevant 'status code' (e.g. HK, HL) to be shown. The underlying status code for UU is HL, however I would not rely on the hope that all business logic treats UU the same as HL.

The eUpgrade system is supposed to auto-acknowledge UU to HL, KK to HK, and KL to HK but sometimes does not.