Outsiders get what ever armor proficiency is "described" for their race.
Only Description I've seen for Neraphim:
Neraphim also prefer to wear strangely colorful body-covering bone spiked leather
What Armor Proficiency do Neraphim get?
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Sign up to join this communityAs per the Neraphim description in the Planar Handbook (p. 12-13 reproduced on the Wizards archive), the Neraphim get no armor proficiencies just for being Neraphim.
But they do get +2 to AC from natural armor, as per the same entry, and can take armor proficiencies from classes or feats as normal.
To quote your question:
Outsiders get what ever armor proficiency is "described" for their race.
I assume this statement is based on the SRD's description of the outsider type (emphasis added):
Proficient with whatever type of armor (light, medium, or heavy) it is described as wearing, as well as all lighter types. Outsiders not indicated as wearing armor are not proficient with armor.
This means that if the example stat block included with the neraphim's creature entry says that it has armor as part of its equipment (which would be calculated into its Armor Class), then that example creature has whatever armor proficiency would be required to wear that kind of armor.
However, if you're creating a character with class levels and assigning the neraphim race, then they get whatever armor proficiency is normally provided by their class.
You didn’t complete your quote:
neraphim also prefer to wear colorful bone-spiked leather (made of stabilized and cured chaos beast hide).
(Planar Handbook, neraphim physical description, pg. 12
The description of neraphim describes them as wearing a type of hide armor, which is a type of light armor.
Neraphim have the outsider type, which means they have all the traits of the outsider type, which means they have this trait:
Proficient with whatever type of armor (light, medium, or heavy) it is described as wearing, as well as all lighter types.
Since neraphim are described as wearing hide armor, a light armor, neraphim are proficient in all armors of that “type” (light), and would be proficient in any armors lighter than that, if there were any.
It isn’t relevant that this goes unmentioned in the neraph racial traits listed in the Planar Handbook. It is not necessary for racial features to spell out all the benefits of standardized features like type; unless something says otherwise (as, for example, with warforged; cf. Eberron Campaign Setting pg. 21), you get everything associated with your type without requiring the racial traits to say anything beyond the fact that you have the type. For the outsider type, that includes a number of proficiencies (neraphim are also proficient with all martial weapons from their outsider type).