E.g.

<ng-content select="[item]"></ng-content>
<ng-content></ng-content>
<ng-content select="a[item]"></ng-content>

Here, we would first find nodes that fit the a[item] selector.
Then we would use the remaining nodes and filter out the ones that match [item] selector.
And only the nodes that don't match any other selector would go into the <ng-content> without a selector.

With this, projection is independent of the order of <ng-content> elements.