The "Communication CPU to Application CPU Interface" (CAIF) is a packet
based connection-oriented MUX protocol developed by ST-Ericsson for use
with its modems. It is accessed from user space as sockets (PF_CAIF).

Say Y (or M) here if you build for a phone product (e.g. Android or
MeeGo) that uses CAIF as transport. If unsure say N.

If you select to build it as module then CAIF_NETDEV also needs to be
built as a module. You will also need to say Y (or M) to any CAIF
physical devices that your platform requires.

See Documentation/networking/caif for a further explanation on how to
use and configure CAIF.

source code:

net/caif/