It's complicated and not necessarily consistent. I'm not even sure I understand what PCGS' policy really is as it seems inhrently inconsistent but copied below.
https://www.pcgs.com/restoration
Submitting a coin for PCGS Restoration in no way guarantees the upgrade of your coin. If the coin is already in a PCGS holder, it is guaranteed not to downgrade, nothing more. Through our restoration service, PCGS is committed to preserving and restoring coins to their original and natural condition.
Toning
PCGS Restoration will not remove original toning simply to make a coin white.
PCGS Restoration will never add toning to a coin.
In cases where PCGS determines that removing toning is necessary and desirable and will not detract from the coin, it may be done. This type of toning may be the result of improper storage, interaction with corrosive materials, or a stain due to an accidental spill.
Surface Contaminants
PCGS Restoration will remove any contaminants that are determined to have been deliberately and artificially applied. Examples include putty, dumdum, grease and other substances known to be used by “coin doctors.”
Restoration will remove contaminants introduced to the coin's surface through improper care or storage. Examples include PVC or Verdigris.
Dirt or debris can often be removed through a gentle rinse.
PCGS Restoration will never add any substance to the surface of a coin.
Spots
Spots on coins can be the result of a variety of causes. Many spots can be removed, yet some cannot be removed without damaging the coin.
PCGS will not attempt to remove a spot unless it is determined that the spot significantly detracts from the coin's overall appearance or is active and will continue to grow.
PCGS will not attempt to restore a spotted coin if that in our opinion, a spot is so deep that removal would lower the coin's overall condition.
Older spots such as old spittle or old thumbprints often cannot be removed without stripping the coin's original surface and will not be attempted.
Newer contaminants such as recent fingerprints can be removed. Also, certain spots due to corrosive interactions can often be removed without impairing the surface.
Metal
PCGS Restoration will never add, remove or alter metal. This includes engaging in well-known “coin doctoring” practices such as smoothing, whizzing, polishing, engraving, etc.
Issues such as scratches, dings, planchet flaws or bending will not be addressed. If it is determined that a coin suffering from one or more of these problems does not have other issues that can be addressed through Restoration, the coin will be returned as is.