The hub normal is calculated according to the settings on the Hubs tab of the Preferences window:
The hub preferences apply to all hubs in the current base polyderon; to change an individual hub normal use the Hub Context Menu instead.
The above animation shows various normal calculations on a superellipsoid envelope. Note that the Envelope normal is almost identical to the Micro-trifan weighted facet average normal.
Because a hub is built around its normal, changing the hub normal alters the alignment of the hub, and consequently the end planes of the struts intersecting with it:
Above: the angle between a hub normal and a strut is marked alpha. The mitre angle for the strut is 180° - alpha.