Besides a letter and number, there are distinct differences between the N95, N99, and R95 face masks which make each better suited to different situations.
An N95 respirator filters 95% of particulate matter from the air but is not resistant to oils. This means that 95% of matter in liquids which would come in contact with the part of the face covered by the mask will not make it through the mask’s respirator, but it is not effective at filtering heavier substances like oil-based substances.
Similar to the N95, an N99 respirator will filter 99% of particulate matter but do not offer protection from oil-based substances. Also like the N95 mask, a properly close fit creates a seal that is more protective than surgical face masks. However, they are still not a complete guarantee against catching coronavirus.
The R95 respirator sets itself apart from the N95 and N99 by not only filtering 95% of particulate matter but also being “somewhat resistant to oil,” according to the CDC. This leg up over the N95 and N99 only means so much, depending on the environment being worked in and what kind of matter is expected to be in the air.
Regardless of these distinctions, the CDC has recommended that the public leave respirators for healthcare professionals, as they are in short supply. Either way, it’s still important for people to actually wear masks in public.