What are "data-url" and "data-key" attributes of tag?

What are "data-url" and "data-key" attributes of tag?

I've faced two strange attributes of an html tag . They are "data-url" and
"data-key".
What are they and how can they be used?
For some reasons i can't show the exact example of the HTML file I've
found them in, but here are some examples from the web with such tags:
data-key
data-key
data-url
PS: I've tried to Google, but no useful results were found.