I honestly do not think it matters, the UMC has a dedicated ground lead, so it's not doing anything funky like an older toaster might do which only has 2 pins on its AC plug. I put the neutral on the "Y" connection of the NEMA 14 receptacle.
Pin/wiring configuration I did:
Y: 277V Neutral
X: 277V Hot
Ground: Ground
Neutral: Ground
Yeah, I ask as on the Wall Connector it definitely matters. You actually have to flip a dip switch to use a line/neutral connection instead of a line to line connection.
The UMC inherently needs to accept a line to line OR a line to neutral connection as they have adapters for both types of connections.
I would not be surprised if it does matter which is the neutral and you just got lucky. It was a 50/50 chance (or I might be wrong and maybe it does not matter).
Thanks for the report!