The short answer is:
we don't know exactly, because Google hasn't publicly documented the `si` parameter. But we can infer quite a bit from how it behaves and how similar tracking systems work.
A shared link looks like this:
```
https://youtu.be/dQw4w9WgXcQ?si=AbCdEfGhIjKlMnOp```
The only part needed to find the video is:
```
https://youtu.be/dQw4w9WgXcQ```
The `?si=…` part can be deleted and the video still works. ([Heise][1])
### What is `si`?
It's almost certainly a
share identifier (or source identifier).
Think of it like a unique ticket in YouTube's database.
Instead of the URL containing your account ID, it contains something like:
```
si=AbCdEfGhIjKlMnOp
```
Internally YouTube might have a database entry like:
| Share ID | Shared by | Time | Platform |
|
---- |
-- |
-– | -—- |
| AbCdEfGhIjKlMnOp | User 12345 | 14:32 UTC | Android app |
The random string itself isn't meaningful to anyone except Google's servers.
### What happens when someone clicks it?
When the recipient opens the link, their browser sends:
```
GET /watch?v=dQw4w9WgXcQ&si=AbCdEfGhIjKlMnOp
```
YouTube then looks up:
> "Ah, this is the share link generated by User 12345."Now it can record things like:
* this share link was opened
* how many times it was opened
* when it was opened
* what country/IP it came from
* what device opened it
* whether the viewer was logged into Google
* whether the viewer watched the video
That's standard web analytics. ([Learn With Parth][2])
### Can YouTube identify the recipient?
If the recipient is:
* logged into YouTube or another Google service →
yes, Google can associate the click with that account.
* not logged in → Google can still see things like IP address, browser fingerprint, cookies, etc., but the identification is less direct.
### Can they tell who shared it?
Yes—that's the whole point of having a unique share ID.
If
you generated the link, YouTube knows that
your account created that particular `si` token.
### Can they map social relationships?
Potentially, yes.
Suppose:
* Alice generates an `si` link.
* Bob clicks it while logged into Google.
Google now knows:
* Alice shared Video X.
* Bob opened Alice's shared link.
Over millions of shares, this can provide information about how content spreads between people and which shares are effective. Google hasn't publicly said it uses `si` to build a "social graph," so that specific claim goes beyond the available evidence. However, the parameter clearly enables attribution of clicks back to a specific sharing event, which supports analytics about sharing behavior. ([Learn With Parth][2])
### Does the `si` string itself contain your account ID?
Probably
not.
It's much more likely to be a random identifier like:
```
AbCdEfGhIjKlMnOp
```
that corresponds to a database record. Without access to Google's backend, someone else can't decode it into "this was shared by Alice."
### Should you remove it?
If you don't want YouTube to attribute clicks back to your specific share, you can safely remove everything from `?si=` onward before sending the link:
```
https://youtu.be/dQw4w9WgXcQ?si=AbCdEfGhIjKlMnOp```
becomes
```
https://youtu.be/dQw4w9WgXcQ```
The video still opens normally in almost all cases. ([Heise][1])
So, the evidence-supported view is:
*
Known: `si` is a unique identifier added to shared links and isn't required to watch the video.
*
Known: It lets YouTube attribute activity to a specific sharing event.
*
Likely: It records who created the share, when it was created, and whether recipients clicked it.
*
Unknown: The exact internal data model and all the ways Google uses the resulting analytics, because Google hasn't published that information.
[1]:
https://www.heise.de/news/Youtube-Video-teilen-Links-Seit-einiger-Zeit-mit-Tracking-Parameter-9615603.html?utm_source=chatgpt.com "Youtube: \"Video teilen\" erstellt Links mit potenziellem Tracking-Parameter | heise online"
[2]:
https://www.learnwithparth.com/blog/why-youtube-appends-si-to-your-links-and-how-to-remove-it?utm_source=chatgpt.com "Why YouTube Appends “?si=” to Your Links & How to Remove It | Learn With Parth"