5.1.1 Multiple logical channel subsystems The following objects are always reserved in the z15 HSA during POR, whether they are  defined in the IOCDS for use: In the z/Architecture, a  single channel subsystem  can have up to 256 channel paths that are  defined, which limited the total numbers of I/O connectivity on older Z servers to 256.  The introduction of  multiple LCSSs  enabled an IBM Z server to have more than one channel  subsystems logically, while each logical channel subsystem maintains the same manner of  I/O processing. Also, a logical partition (LPAR) is now attached to a specific logical channel  subsystem, which makes the extension of multiple logical channel subsystems not apparent  to the operating systems and applications. The multiple image facility (MIF) in the structure  enables resource sharing across LPARs within a single LCSS or across the LCSSs.  The multiple LCSS structure extended the Z servers’ total number of I/O connectivity to  support a balanced configuration for the growth of processor and I/O capabilities.  A one-digit number ID starting from 0 (CSSID) is assigned to an LCSS, and a one-digit  hexadecimal ID (MIF ID) starting from 0 is assigned to an LPAR within the LCSS. Note:  The phrase  channel subsystem  has same meaning as  logical channel subsystem  in  this section, unless otherwise stated. A  subchannel  provides the logical appearance of a device to the program and contains the  information that is required for sustaining a single I/O operation. Each device is accessible by  using one subchannel in a channel subsystem to which it is assigned according to the active  IOCDS of the Z server.  A subchannel set (SS) is a collection of subchannels within a channel subsystem. The  maximum number of subchannels of a subchannel set determines how many devices are  accessible to a channel subsystem. In z/Architecture, the first subchannel set of an LCSS can have 63.75 K subchannels (with  0.25 K reserved), with a subchannel set ID (SSID) of 0. By enabling the multiple subchannel  sets, extra subchannel sets are available to increase the device addressability of a channel  subsystem. For more information about multiple subchannel sets, see 5.1.2, “Multiple  subchannel sets” on page 208. Subchannels GLYPH<SM590000> Six CSSs GLYPH<SM590000> A total of 15 LPARs in each CSS0 to CSS4 GLYPH<SM590000> A total of 10 LPARs in CSS5 GLYPH<SM590000> Subchannel set 0 with 63.75 K devices in each CSS GLYPH<SM590000> Subchannel set 1 with 64 K minus one device in each CSS GLYPH<SM590000> Subchannel set 2 with 64 K minus one device in each CSS GLYPH<SM590000> Subchannel set 3 with 64 K minus one device in each CSS Chapter 5. Central processor complex channel subsystem  207