When I set up my robot, I mapped UI[1-3] & UI[8] to the same physical input point, and let the PLC control all 4 at once with a single output.
As far as what each rack is, this is a list that somebody posted on this forum a few years ago:
QuoteDisplay MoreRack:
0 process I/O boards (also memory image)
16 AB or Genius I/O
32 Slave SLC2 I/O
33 internal relay/register
34 flag marker
35 always on/off port Slot 0 = OFF Slot 1 = ON
36 DCS port
48 address mapped I/O
64 ME-NET
65 INTERBUS-S
66 PROFIBUS DP master
67 PROFIBUS DP slave
68 FL-net
69 FL-net status
70 InterBus-S master
71 InterBus-S slave
72 IO-LINK II master
73 IO-LINK II slave
74 FIPIO master
75 FIPIO slave
81 first DeviceNet board
82 Used by DeviceNet
83 Used by DeviceNet
84 Used by DeviceNet
85 controlnet; also used 86
86 Used by ControlNet
87 RoboWeld
88 Ethernet Global Data (GE-EGD I/O)
89 EthernetIP (ControlNet over ethernet) I/O
90 Arclink Rack Number
91 WTC Serial Weld Controller I/O
92 CC-Link RD
93 InterBus PxC PCI master
94 InterBus PxC PCI slave
95 InterBus PxC PCI cmd
96 Modbus TCP
97 TOYOPUC PC3J Interface
98 InterBus PxC Slave interface
99 PROFINET I/O Controller
100 PROFINET I/O Device