Via tenting is done using dry film soldermask to cover the hole (either in a single process with the rest of the board or as by applying cover dots before another soldermask pass). Via plugging is achieved by pressing liquid soldermask into the vias using a squeegee and then applying soldermask (dry film or liquid) to the rest of the board.
Whether the via’s hole is reliably tented or plugged, though, depends on the size of the barrel, the thickness and viscosity of the soldermask, how soldermask is applied, and the effectiveness of the curing process. Therefore, across the many variables there is no guarantee that a hole will be appropriately covered. Crucially, tenting and plugging vias may entrap air, debris, solvents, contaminants, and other materials from the manufacturing process that cannot be removed in subsequent cleanup stages. Entrapped elements can cause eventual defects in the barrel, but also blow-outs when the trapped gasses expand during curing and soldering. These blow-outs can result in soldermask teardown, exposing the copper to oxidation and contamination, which may lead to defects in the short- and long-terms. In fact, some companies forbid tented and plugged vias altogether for this reason.
It’s sometimes helpful to place a via just a pad ― this is called via-in-pad ― or very close to one where there cannot be any soldermask in between the pad and the via. This may be good for signal integrity, tricky signal fanout, or tight routing spots. If we do that without consideration, though, the via opening creates a nonuniform pad surface and can wick the solderpaste that’s used for soldering components; both of these things cause bad solder joints. Designers, therefore, choose via filling where the via is filled and only then a uniform pad is manufactured on top of it. Filled vias can be capped with copper and covered with soldermask.