> 'Additive' here means that if u(K1) is defined as the unknotting number of the knot K1, and u(K1#K2) the unknotting number of the knots K1 and K2 joined together, then u(K1#K2) ≤ u(K1) + u(K2).

Kinda like the triangle inequality[1] of knots?

I recall the triangle inequality was useful for several cases in Uni, if so I guess I can see it might be a similarity useful inequality in knot theory.

[1]: https://en.wikipedia.org/wiki/Triangle_inequality

I incorrectly had a ≤ instead of =, my apologies.