It sounds like you're thinking of reification. From Cambridge Dictionary:

reification
noun [ U ] /ˌriː.ɪ.fɪˈkeɪ.ʃən/ /ˌriː.ɪ.fɪˈkeɪ.ʃən/ formal ​

the act of changing something abstract (= existing as a thought or idea) into something real:
the reification of fantasies

This standard definition is very slightly different from an existing object that stands in for something that remains abstract (that sounds a bit more like a symbol), but I can see it being used that way in a specialized context. I've mostly heard it used in the social sciences, but Wikipedia says that in computer science

Reification is the process by which an abstract idea about a computer program is turned into an explicit data model or other object created in a programming language. A computable/addressable object — a resource — is created in a system as a proxy for a non computable/addressable object.

That second sentence, especially, sounds like what you've described.

mathom /ˈmæðəm/

A lovely, ancient, and eminently modern word for this is mathom. The OED gives for the current sense of mathom:

2. A trinket, a piece of bric-a-brac.

But you need to know more about it that just that, for there’s more to it than just that.

Its original sense from Old English is now obsolete:

1. A precious thing, a treasure, a valuable gift. Obsolete

For its origin OED notes that:

Etymology: Cognate with Old Saxon mēðom (masculine) precious thing, a treasure, Old Icelandic meiðmar (feminine plural) valuables, Gothic maiþms (masculine) gift, ultimately < the Indo-European base of classical Latin mūtāre to exchange (see mutate v.).

The word was playfully revived in a slightly different sense (see sense 2) in the 20th. cent. by J. R. R. Tolkien.

Here’s a Middle English citation:

• c1275 (▸?a1200) Laȝamon Brut (Calig.) (1963) 896
  Ȝeue us þe king & al his gold, & þa maðmes of his lond.

In other words, “Give us the king and all his gold, and the mathoms of his land.” They were after treasure.

Tolkien’s “playful revival” of the word in a slightly different sense appears first in the Prologue to The Lord of the Rings, where he wrote:

At no time had Hobbits of any kind been warlike, and they had never fought among themselves. In olden days they had, of course, been often obliged to fight to maintain themselves in a hard world; but in Bilbo's time that was very ancient history. The last battle, before this story opens, and indeed the only one that had ever been fought within the borders of the Shire, was beyond living memory: the Battle of Greenfields, S.R. 1147, in which Bandobras Took routed an invasion of Orcs. Even the weathers had grown milder, and the wolves that had once come ravening out of the North in bitter white winters were now only a grandfather's tale. So, though there was still some store of weapons in the Shire, these were used mostly as trophies, hanging above hearths or on walls, or gathered into the museum at Michel Delving. The Mathom-house it was called; for anything that Hobbits had no immediate use for, but were unwilling to throw away, they called a mathom. Their dwellings were apt to become rather crowded with mathoms, and many of the presents that passed from hand to hand were of that sort.

And it has been used by other authors, in non-Hobbitty contexts, since then. For example, the OED provides (among others) a citation from Byte magazine:

• 1998 Byte Jan. 123/1
  A storage company where I keep a bunch of mathoms—stuff I can't quite bring myself to throw away.

See also the LotR Wiki entry for this word, which begins with:

Mathom was the hobbit term for anything which they had no use for but were unwilling to throw away. Their holes and houses usually were quite crowded with mathoms. Hobbits were very fond of giving mathoms to one another; on birthdays, it was tradition that the hobbit who had the birthday would give a gift to anyone who attended his party. This way mathoms travelled from hand to hand often around the whole Shire, sometimes finding their way back to the original owner. Weapons and other gear of war was usually looked upon as mathoms in the Shire and usually they became trophies hanging over fireplaces or on walls.