Informative mobile scrolling by tag or category

A designer came to one of our iOS engineers to show us some inspirations she saw with some iOS apps where they allow users to scroll by tag or category Some examples include Snapchat where if you go to your Stories list and to the right you see a purple scrollbar, if you click anywhere it will not only jump the location but also popup what index of the scroll you're looking at (it sorts snaps based on recency and then alphabetical contacts).

I believe there were some other iOS apps that do this i.e. Dropbox but I wasn't entirely sure what this UX feature would be called? Scrolling with tags? Scrolling by category?

I'll try to add a picture when I can.