Dapper's Query 'map' and 'splitOn' parameters

Program.cs

using Dapper;
using Microsoft.Data.Sqlite;
using Newtonsoft.Json;

namespace Dappering
{
    class Program
    {
        static void Main(string[] args)
        {
            using var db = new SqliteConnection("Data Source=:memory:");
            db.Open();

            db.Execute(@"
                CREATE TABLE Customers (CustomerID Integer Primary Key, Name Text);
                INSERT INTO Customers VALUES (1, 'A'), (2, 'B');
                
                CREATE TABLE Orders (OrderID Integer Primary Key, OrderNumber Text, CustomerId Integer);
                INSERT INTO Orders (OrderNumber, CustomerID) 
                VALUES ('O1', 1),
                    ('O2', 2),
                    ('O3', 2),
                    ('O4', 2);");

            var orders = db.Query<Customer, Order, Order>(
                sql: @"SELECT c.CustomerID, Name, OrderID, OrderNumber
                        FROM Customers c
                        JOIN Orders o ON o.CustomerID = c.CustomerID",
                map: (customer, order) =>
                {
                    order.CustomerName = customer.Name;
                    return order;
                },
                splitOn: "CustomerID,OrderID");

            System.Console.WriteLine(JsonConvert.SerializeObject(orders, Formatting.Indented));

            /*
            [
                {
                "OrderNumber": "O1",
                "CustomerName": "A"
                },
                {
                "OrderNumber": "O2",
                "CustomerName": "B"
                },
                {
                "OrderNumber": "O3",
                "CustomerName": "B"
                },
                {
                "OrderNumber": "O4",
                "CustomerName": "B"
                }
            ]
            */
        }
    }

    public class Customer
    {
        public string Name { get; set; }
    }

    public class Order
    {
        public string OrderNumber { get; set; }
        public string CustomerName { get; set; }
    }
}