What is +U1 in email username?

This is NOT a Win11 problem per se, but as the post title suggests, I received an email with a+U1 in the username
Example:
username+U1@email.com
vs
username@email.com

I was unable to find anything on the interweb about it.. so posting here to see if anyone can share any info/knopwledge.

Regards,

Could just be something like this:



I use this feature to help filter some spam.

RickyMoose said:

Could just be something like this:

2 hidden ways to get more from your Gmail address

Posted by Robby Stein, Associate Product Marketing Manager I recently discovered some little-known ways to use your Gmail address that can g...

gmail.googleblog.com

I use this feature to help filter some spam.

Click to expand...

Thanks.. you may be right.. wonder why that never surfaced during my search..