Very much this.

I would think that some people might dig their nails into their skin on rare occasions when presented with a lot of stress. However, if you're doing it regularly and enough for your friends to notice, I would think it is a form of self-harm. Not all self-harm involves drawing blood. The intent is simply to cause harm to yourself for a variety of reasons (such as distracting your mind from the emotional pain or a desire to feel real).

When I can't cut for whatever reason, I either dig my nails into my skin or scratch myself. Sometimes, it leaves marks that last for 1-2 days. If you're concerned about it, I recommend speaking to someone about it. I don't want to say any form of self harm is better than another. As while physically, one might be 'safer'... emotionally, they can all be quite hard to cope with.

I wish you all the best.