Does any one know what the AERS horse registration is. ?

I have a chance to buy an Arabian mare that is registered half arabian. Her father is registered with AHR and her mother AERS but i really don't know what that is.
Update: I have a copy of her papers and her father is registered with AHR and has a list of his ancestors but under dam it says her name and under her name is AERS her registration number but none of her ancestors
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