What is a RAID Controller's Sideband Cable used for?
The sideband cables are used to connect the controller to a managed backplane or disk enclosure, so that it can monitor the controller as well as the disks.
This PDF document "SFF-8448 Specification for SAS Sideband Signal Assignments", (Rev. 05; September 2, 2005) on Seagate's FTP Server defines the signal assignments for known Vendor specific sidebands thata are defined in SAS (Serial Attached SCSI).
On page 12 there is a description of the Sideband Assignments.