All you need do to sort the top level of a subtree is to select the root node of the subtree you want to sort, then press Shift+Ctrl+S to sort it!
Before sorting, in the example below, both B and B1's subtrees are not in ascending order (A-Z from top to bottom). After sorting, B's subtree is sorted but B1's subtree is not, since only the top level was sorted.
To sort B's subtree at all levels, you need to use one of the menu commands such as Tree context menu ⁄ Sort ⁄ Subtree (all levels).
B's subtree sorted (all levels)