Open your manual to the "Technical Specifications" section. It does not mention that Intel boards of this era are notorious for leaking electrolytic capacitors (often near the CPU socket labeled "21 B6 E1"). If your PC randomly shuts down or fails to start, the manual will tell you to check the PSU. The better advice: Inspect for bulging, rusty-topped capacitors.
Before you can use a manual, you need the right one. Searching for "21 B6 E1 E2" verbatim will lead you to dead ends because that is not a product name. intel desktop board 21 b6 e1 e2 manual better
You have the document. Now, let's make your vintage Intel board actually useful. Open your manual to the "Technical Specifications" section
The Intel manual (PDF) tells you what each BIOS setting does but not why you’d change it. It lacks: You have the document