Metal or leather or combination gauntlets, usually not covering the hands and fingers, but just the forearms which immobilize Sime's tentacles and prevent transfer or the Kill. They prevent the use of the handling tentacles, thus inflicting a sense of being crippled on the Sime. 

When wearing them a Sime cannot sense selyn field gradients properly. They are also painful to wear, especially for longer times. 

They were invented by Gens wanting to immobilize and question Sime prisoners.  They evolved into a torture device, and after Unity, the treaty negotiated the requirement that Simes out-Territory must wear retainers.

During Unity, a series of redesigns were authorized, and various businesses founded to create retainers that were custom made with interior designs that allowed them to be worn longer.  Each change was, of course, a political crisis of worldwide proportions.

After Mairis Farris introduced the Attenuator, attitudes changed very quickly.  The Attenuator worked to the Sime's advantage, shielding the Sime from unwanted stumulus with adjustable fields, almost the obverse of retainers that worked to shield the Gens from irrational phobia or actual fear.

From there it was only a short step to the abolition of Territory borders in law if not in fact.

(Source:  Index card file, SIME SURGEON "gleaming metal gauntlets" encased forearms half way from wrist to elbow.  Tentacles lay under the retainers.  Digen risked being shot on sight if he didn't wear them in Gen Territory.  Retainers are individually tailored for maximum comfort and minimum inpairment.  The Gen police retainers are heavy and cruel, black metal, almost like those used for torturing sime prisoners during the Sime/Gen Wars -- have pressure bars inside, meant to press on the nerve plexuses of the laterals, handling tentacles extended.

Wearing retainers while in Need (gland swelling, pressure bar presses on gland and nerve) can lead to paralysis (in Digen, anyway!) and unconsciousness, and eventually, death. They don't really protect Gens -- a sime intent on Killing can just take them off and attack.

(Source:  Index card file, OPERATION HIGH TIME  --  Invented by Gens about 400 years previously, so that simes captured in the Sime Wars couldn't attack and Kill them.  Caused bruised soreness and general weakening of tentacles after constant use.  They provide discomfort by cutting off the selyn field sensations of Simes.  Retainers can cause pain, pinching tentacles (laterals) often a problem that simes can't sense fields with them on.  Causes disorientation.

(Source:  Index card file, UNTO ZEOR, FOREVER)  Invented by Gens during the Sime/Gen Wars to restrain and torture simes to reveal military information.  Not too much changed since then.  Material laminated between outer walls interfered with sime sensory system.  Interior pockets and bars forced the tentacles into extension and immobilized them painfully. Hard to take off -- must move slowly to avoid rush of sensation, stiff muscles, loud clasp sound when opening the retainers, which opens the seals along the inside of forearms.  Inside construction is MEANT to make them hard to remove.

Retainers are bulky.  Digen needed to have uniforms altered.  Makes a "sickening blur" of the world.  Out-Territory Gens could legally murder a retainerless sime out-Territory.  Wearing retainers interfers with the sime's entire perceptual system, producing symptoms of sensory deprivation if worn long enough.  Gnarled ridges of tentacle sheaths visible above the tops of the retainers.