Applies to ListBox and MultiSelector, which covers ListView and DataGrid. It exists for one reason: Selector.SelectedItems is read-only, so it cannot be bound.

<ListBox SelectionMode="Extended"
         ItemsSource="{Binding People}"
         mah:MultiSelectorHelper.SelectedItems="{Binding SelectedPeople}" />

The binding runs both ways: selecting items in the list adds them to your collection, and adding to your collection selects them in the list.

The view model side wants a collection that raises change notifications, so an ObservableCollection<T>:

public ObservableCollection<Person> SelectedPeople { get; } = new();

Give the property a getter only and fill the existing collection rather than replacing it. Replacing the instance means the helper is still watching the old one.

SelectedItemBinding is used internally to hold the binding while it is attached; it is not something to set.

Related

SelectionMode is still WPF's own — the helper synchronises whatever the list allows, so a ListBox left at Single will only ever put one item in your collection.