What does vacuum mean in this fuse diagram? (1999 chevy express 1500

1999 Chevy Express Fuse Diagram

Vacuum fuse What does vacuum mean in this fuse diagram? (1999 chevy express 1500

What does vacuum mean in this fuse diagram? (1999 chevy express 1500

What does vacuum mean in this fuse diagram? (1999 chevy express 1500
What does vacuum mean in this fuse diagram? (1999 chevy express 1500