How to designate a dynamic property name?

Question

API

Hello. I am writing documentation in the form of openAPI 3.0 The
request creates an Excel file with data. The data is transmitted in this way

{
  "document_name": "Document Name",
  "general_count": 1,
  "data": {
    "fields": {
        "fields": {
            "A": "какое то значение"
        },
        "string_number": 10
    }
  }
}

Where in fileds "A" is the name of the column in excel. But since there are a lot of columns in Excel, all the letters are listed in the documentation, well, like this. How to make it clear that the property name is dynamic?