Overnight inventory is shorthand for the directional character of the electronic session before regular trading hours begin. It can describe a market that spent most of its time above, below, or around the prior cash close.
It is not a list of every trader position. It is a read of where the overnight auction accumulated and how extended it became before the cash open.
The US open can confirm, rebalance, or reverse that inventory. That is why the label is useful context and unsafe as a standalone forecast.