Luthiers often refer to an epoxy pore filler they have to sand and steel wool in multiple coatings to make an even, level surface for applying a high-gloss glass-smooth finish. Which is also applied in layers and sanded and buffed.
I speculate that instead of matching the surrounding wood color, they use a contrasting colored epoxy to make the pores and grain pop?
But I'll defer to the people who work with the materials to provide an informed answer.
Kamaka uses an open-pore finish on their ukes. Just enough to seal the wood, while allowing the pores to show.
Pono, on the other hand, puts a glass smooth, high-gloss finish on their deluxe and PC models.
Mya-Moe uses a multi-layered True Oil finish on their instruments. (Also used on gun stocks.) Including on the fretboard.