That would sort the nodes after the value you have assigned to them. When you set sortmode="name" then it will be sorted alphabetically. The sort group can be used to let some types of nodes always appear at the top, e.g. in a folder view you would want to display the folders at the top.
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sortmode can be "value", "name" or "extension"
You basically assign a value to each entry adn sort by value.
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