It's a remarkably flexible layout manager. Full width components at the top and bottom using their preferred height, full (remaining) height components on either side using their preferred width, and the rest.
You can, of course, define a layout behaviour that has more panels. But that would be a different layout. There's nothing stopping people writing one (and many have), but it doesn't make a BorderLayout less useful.
(The only layout manager I've ever written myself that I actually found useful was an "inverse border layout". A centred component of it's own preferred size, and then components filling the spaces round the edges. it came in handy for interfaces with two lists of the same size with some buttons in the middle for transferring between them).
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