I've noticed this tag in the themes that ship with wordpress, but in spite of rooting through both Codex and the template function files I can't figure out what it does and why it might be necessary. All I've found is this:
Always passed 1 as a parameter. Never executes in current code.
which leaves me none the wiser. So does anyone know why it's there, and whether I need to include it in any themes I might make?