In Tenet, the Protagonist was saved by a mysterious shooter who had a trinket tied with an orange (or red) string to their backpack. The Protagonist later encounters the same trinket and string on an inverted person's backpack as he was trying to prevent the time capsule from being buried. Near the end of the film, we learn that the mysterious person with the trinket tied to their backpack was actually Neil.
What is the trinket on Neil's backpack? It looks like a plain washer to me, but I'm wondering if it's actually an object with a deeper symbolism or significance. Does the trinket or string have symbolism or significance connected to the movie's themes, or about Neil's character?
When you're in a military unit and you all have the same gear loaded onto trucks and stuff, it can be hard to pick out your green backpack from the other sometimes dozens of exact same bags, so you'd have a tag or trinket or marking that would individualize it.
In terms of significance in the movie - it's just a nice way for the Protagonist and the audience to (later) recognise the individual who intervened to help at the opera house.