Get a headshot Kill at 500+ metres Assignments Challenge completed

Hi !

I juste made a long distance headshot kill and I get this notification on the left side of my screen :

ASSIGNMENTS CHALLENGE COMPLETED
Get a headshot Kill at 500+ metres
1/1

(see capture)

But I cant find this "Assignments Challenge" and this reward anywhere in game menu...
What does this mean ?
What is this challenge ?