It is absent in the Ginger Payments API documentation, but is used by the official Ginger Payments extension for Magento 1.x.
I have found a JSON schema of it:
{
"type": "object",
"properties": {
"merchant_customer_id": {
"description": "Merchant's internal customer identifier",
"type": ["string", "null"]
},
"email_address": {
"type": ["string", "null"],
"format": "email"
},
"first_name": {
"type": ["string", "null"]
},
"last_name": {
"type": ["string", "null"]
},
"address_type": {
"type": ["string", "null"],
"enum": ["customer", "delivery", "billing"]
},
"address": {
"type": ["string", "null"]
},
"postal_code": {
"type": ["string", "null"]
},
"housenumber": {
"type": ["string", "null"]
},
"country": {
"description": "ISO 3166-1 alpha-2 country code",
"type": ["string", "null"],
"pattern": "^[a-zA-Z]{2}$"
},
"phone_numbers": {
"type": ["array", "null"],
"items": { "type": "string" }
}
},
"additionalProperties": false
}