I found the article Arco capaz on the Spanish Wikipedia site, and from what I understand, an arco capaz is the locus of all points P from two distinct fixed points A and B such that the angle APB is a fixed angle.
I don’t think there is a special name for this arc in English. Clearly the locus is an arc of a circle, so we just call it that in English. If the arc is smaller than a semicircle (i.e. if is greater than 90°) it is called a minor arc; if it is larger than a semicircle (i.e. the fixed angle is acute) it is called a major arc.
Espero que esto te ayude.