UA is not premium economy, period. You can not buy a premium economy ticket on UA. Economy Plus is only extra legroom that people may choose to pay extra for access to or UA gives free to their own elites. Being sold as a unique class of service would be a logical bare minimum to be called premium economy I'd think. Generally though it also means a bit of a wider seat, better meal or wine offerings over standard economy, etc. China is the only place I can think of off hand where they actually try to sell more legroom with standard service as premium economy (CA & CZ both, unsure about MU & HU)