PCB fabricators may request permission to apply Soldermask Barrel Reliefs to your tented vias when the via's hole sizes are 18mil or larger in size. Consult with your fabricator as the requirements vary between suppliers.
For more about Soldermask Barrel Reliefs see : Via Barrel Reliefs - PCB Designer Blog
Fabricators specify the barrel reliefs with respect to the via hole size. For example drill hole size + 5mil is typical.
The +5mil soldermask relief accounts for registration and process tolerances.
To Force Complete Tenting
Tenting vias on one side can trap chemicals and fluxes in the via barrels which can corrode away the copper plating in the via barrels. This is a reliability issue.
Reference: Soldermask on via-holes in case of chemical Nickel-Gold surface finish
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