We update user's contact information from our site, and use the TaxNumber field.
I assume it guesses the TaxNumberType, a field available in the user UI, based on the format of the value passed in. We have people change from using SSN to EIN because their mode of business changes. This throws an error "Social security number must be 9 digits and in the format xxx-xx-xxxx."
This should be a field accepted through the API.
