This defines which facility field is used to carry DCP, a Codex-specific data-compression protocol, information during call setup and reconnection in Call Request and Call Accept packets. This is a low-level X.25 protocol-compatibility setting an admin only touches if it conflicts with another private network's use of the same facility number; otherwise it should be left at its default across a consistent network so all nodes agree on where to find DCP negotiation data in call packets. For example, merging this network with another private X.25 network that happens to use the same facility number for a different purpose would require changing this value to avoid the two networks' call packets colliding on that field.