What does the attach bit in an IS-IS PDU indicate?

What does the attach bit in an IS-IS PDU indicate?

A.
The L1/L2 router is exporting a default route into the area.

B.
The L1/L2 router has reachability information to another IS-IS area.

C.
The L1/L2 router is sending information from Level 2 towards Level 1.

D.
The L1/L2 router is sending information from Level 1 towards Level 2.