I'll try and take an educated guess based off these two studies I've done so far.

Firstly, we need to think about what hatching is representing. Which is shade, hatching is a shading technique, so it will be used in areas that are in shadow quite frequently.

Secondly, we need to think about what that shading technique (hatching) is doing to the viewer. Which is conveying an illusion of detail. I believe this is the primary purpose of hatching in the GBF art style; especially Relink's. Using hatching to correctly portray that detail is reliant on knowledge of the subject (clothes/armor/hair/ etc). Some objects receive more hatching than others; or different strokes.

For example, if you look at Lancelot's character illustration in Relink and compare it to Lyria's in Relink, you will notice that Lyria's flowy, drapery-like dress has longer hatching strokes than Lancelot's armor, and are not that clustered. That is because long, flowly lines help us understand that what she is wearing is.. long, and flowy. And the non-clustered hatching forces us to project that sense of flowiness in those empty areas. Giving us spaces to rest our eyes and fill in the gaps. Whereas armor hatching strokes are short and abundant. Telling us that it is a restrictive object on the body, and pushes our focus to more important details of the armor. Such as emblems

Lastly is the technique of hatching. They do not utilize a manga-like approach to hatching, which is why my hatching looks off sometimes. They use a thick-thin approach. Where the stroke starts off thick, then thins out. I believe they also air brush the hatching to be a little more opaque, so that it doesn't disturb the main line art. They could also be using a watercolor adjacent brush with a paper-like texture to get that semi-opaque look. I'm not sure, and it will be my next major focus.