IBM z13s Technical Manual page 220

Device in subchannel set 0 and additional subchannel sets
An LCSS will always has the first subchannel set (SSID 0), which can have up to 63.75K
subchannels, with 256 subchannels reserved by the channel subsystem. That user can
always define their I/O devices in this subchannel set for general use.
For the additional subchannel sets enabled by the MSS facility, each has 65,535 subchannels
(64K minus one) for specific types of devices. These additional subchannel sets are referred
as alternate subchannel sets in z/OS. Also, a device that is defined in an alternate
subchannel set is considered a special device, which normally will have a special device type
in the I/O configuration as well.
Currently, a z13s server that runs z/OS supports defining these types of devices in an
additional subchannel set, with proper APAR or program temporary fix (PTF) installed:
Alias devices of the parallel access volumes (PAV)
Secondary devices of GDPS Metro Mirror Copy Service (formerly PPRC)
FlashCopy SOURCE and TARGET devices with PTF OA46900
DB2 data backup volumes with PTFOA24142
Using an extra subchannel set for these special devices helps to relieve the numbers of
devices in the subchannel set 0, which improves growth capability for the system.
IPL from an alternate subchannel set
z13s servers support IPL from alternate subchannel sets in addition to subchannel set0.
Devices that are used early during IPL processing now can be accessed by using subchannel
set 1, or subchannel set 2 on a z13s server. This configuration allows the users of Metro
Mirror secondary devices that are defined by using the same device number and a new
device type in an alternate subchannel set to be used for IPL, an I/O definition file (IODF), and
stand-alone memory dump volumes when needed.
IPL from an alternate subchannel set is supported by z/OS V1.13 or later, and Version 1.12
with PTFs.
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IBM z13s Technical Guide