One common usage of signals on the Amiga is for processing a user break.
As was mentioned earlier, the OS reserves 16 of a tasks 32 signals for
system use.  Four of those 16 signals are used to tell a task about the
Control-C, D, E, and F break keys.  An application can process these
signals.  Usually, only CLI-based programs receive these signals because
the Amiga's console handler is about the only user input source that sets
these signals when it sees the Control-C, D, E, and F key presses.

The signal masks for each of these key presses are defined in <dos/dos.h>:

    SIGBREAKF_CTRL_C
    SIGBREAKF_CTRL_D
    SIGBREAKF_CTRL_E
    SIGBREAKF_CTRL_F

Note that these are bit masks and not bit numbers.