The pmac_zilog driver for the z85C30 chip on many powermacs
historically used the device numbers for /dev/ttySn. The
8250 serial port driver also uses these numbers, which means
the two drivers being unable to coexist; you could not use
both z85C30 and 8250 type ports at the same time.

If this option is not selected, the pmac_zilog driver will
use the device numbers allocated for /dev/ttyPZn. This allows
the pmac_zilog and 8250 drivers to co-exist, but may cause
existing userspace setups to break. Programs that need to
access the built-in serial ports on powermacs will need to
be reconfigured to use /dev/ttyPZn instead of /dev/ttySn.

If you enable this option, any z85c30 ports in the system will
be registered as ttyS0 onwards as in the past, and you will be
unable to use the 8250 module for PCMCIA or other 16C550-style
UARTs.

Say N unless you need the z85c30 ports on your (Power)Mac
to appear as /dev/ttySn.