The caption for the image ISS016-E-008937 says

ISS016-E-008937 (3 Nov. 2007) --- While anchored to a foot restraint on the end of the Orbiter Boom Sensor System (OBSS), astronaut Scott Parazynski, STS-120 mission specialist, assesses his repair work as the solar array is fully deployed during the mission's fourth session of extravehicular activity (EVA) while Space Shuttle Discovery is docked with the International Space Station. During the 7-hour, 19-minute spacewalk, Parazynski cut a snagged wire and installed homemade stabilizers designed to strengthen the damaged solar array's structure and stability in the vicinity of the damage. Astronaut Doug Wheelock (out of frame), mission specialist, assisted from the truss by keeping an eye on the distance between Parazynski and the array.

It can also be found in Wikipedia.

Is this cable simply an emergency backup if something breaks, or is one potential use to allow the astronaut to disconnect and move farther from the end of the boom. If so, has it ever been used like that?

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Full size (1.7 MB) if clicked:

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  • $\begingroup$ Related information space.stackexchange.com/a/26283/12102 $\endgroup$ – uhoh Jun 20 '18 at 12:26
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    $\begingroup$ you can also see it in i.stack.imgur.com/qtaG4.jpg included in that answer. I assume it stops the mount from disappearing into the black should the connector fail, or following an inadvertent release. $\endgroup$ – user20636 Jun 20 '18 at 12:44

What you are seeing the cable emerge from is not his boot but rather the reel end of a Body Restraint Tether. The other end would be clipped to the waist restraint of his suit.

This tether can reel in or out cable against a spring force or the reel can be locked. For this particular EVA (repairing the ripped solar array) there was no plan for Parazynski to leave the foot restraint during ops but the tether would not have prevented it.

This tether type was commonly used on all kinds of EVAs. Its purpose is to provide a redundant attachment to structure. In the EVA pictured in the question, if the boot-to-foot-restraint or the foot-restraint-to-OBSS attachment failed, the crewperson would still be attached to the OBSS by the tether.

Front view of Body Restraint Tether ("circled" in red) from this EVA Hardware and Operations Overview

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Here's a similar (perhaps older design) retractable tether in use on the first Shuttle EVA from here.

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  • $\begingroup$ I thought @uhoh was referring to the thin gray cable leading from box under the left heel to the end handhold on the canadarm, not the white one leading from between the toes to the waist $\endgroup$ – user20636 Jun 20 '18 at 22:01
  • $\begingroup$ That is what I am describing as well. The thin wire comes out of the reel. The gray box contains the reel. $\endgroup$ – Organic Marble Jun 20 '18 at 22:03
  • $\begingroup$ ah, so the grey box is at the end of the white tether attached to the waist, and the gray cable isn't (easily?) visible in the cargo bay picture.with only the tether circled in you diagram I didn't quite understand $\endgroup$ – user20636 Jun 20 '18 at 22:20
  • $\begingroup$ Yes, the wire in the payload bay picture comes out of the reel and goes left and down - it's why the reel is pulled away from the crewmember. $\endgroup$ – Organic Marble Jun 20 '18 at 22:23
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    $\begingroup$ (it was the second picture I can't see the cable in) $\endgroup$ – user20636 Jun 20 '18 at 22:28

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