This is in relation to increasing manufacturing reliability when placing vias in/on pads. I believe the best practice is to tent vias that are in pad but I could be wrong. I think there is conflicting guidance depending on who you talk to (pcb manufacturers vs bga/qfn parts makers).
At any rate the concept is to apply solder mask to the via (I think) possibly completely covering it (I think). I can't tell if this is to reduce the capillary action through to the opposite side from where the bga/qfn part sits or if it's for the same side as the bga/qfn part but for a different reason. I think it's the former.
I guess for exposed thermal ground pads on qfn parts it's best to tent.
I have some parts of my design where vias snuggle up close (touching) leads on qfn parts. I'm assuming the same capillary action is vulnerable here as well. I suppose I should attempt to move those vias away from the leads. Just don't know if it's that big of a concern.