What is true about the DLCI field in the Frame Relay header?

What is true about the DLCI field in the Frame Relay header?

A.
It consists of two portions, source and destination, which map data to a logical channel.

B.
It generally has significance only between the local switch and the DTE device.

C.
It is an optional field in the ITU-T specification.

D.
It is present only in data frames sent through the network.

Explanation:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1918.htm#xtocid3