In short ~ use it for non-commercial projects (ie those not designed to make money)………if you are content to add a hyperlink (preferably active) to your creations to here https://kefiweb.com/kefilinkicon/
You are also free to use the initial icon design make your own designs (“derivatives”) ~ again with a link back here.
A derivative could be as simple as adding a shape to the kefilinkicon. Here’s an example of one I made……..