I think it refers to the image model that's made by modelsbuilder.

//------------------------------------------------------------------------------
// <auto-generated>
//   This code was generated by a tool.
//
//    Umbraco.ModelsBuilder v3.0.6.97
//
//   Changes to this file will be lost if the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using System.Web;
using Umbraco.Core.Models;
using Umbraco.Core.Models.PublishedContent;
using Umbraco.Web;
using Umbraco.ModelsBuilder;
using Umbraco.ModelsBuilder.Umbraco;

namespace Umbraco.Web.PublishedContentModels
{
    /// <summary>Image</summary>
    [PublishedContentModel("Image")]
    public partial class Image : PublishedContentModel
    {
#pragma warning disable 0109 // new is redundant
        public new const string ModelTypeAlias = "Image";
        public new const PublishedItemType ModelItemType = PublishedItemType.Media;
#pragma warning restore 0109

        public Image(IPublishedContent content)
            : base(content)
        { }

#pragma warning disable 0109 // new is redundant
        public new static PublishedContentType GetModelContentType()
        {
            return PublishedContentType.Get(ModelItemType, ModelTypeAlias);
        }
#pragma warning restore 0109

        public static PublishedPropertyType GetModelPropertyType<TValue>(Expression<Func<Image, TValue>> selector)
        {
            return PublishedContentModelUtility.GetModelPropertyType(GetModelContentType(), selector);
        }

        ///<summary>
        /// Size
        ///</summary>
        [ImplementPropertyType("umbracoBytes")]
        public string UmbracoBytes
        {
            get { return this.GetPropertyValue<string>("umbracoBytes"); }
        }

        ///<summary>
        /// Type
        ///</summary>
        [ImplementPropertyType("umbracoExtension")]
        public string UmbracoExtension
        {
            get { return this.GetPropertyValue<string>("umbracoExtension"); }
        }

        ///<summary>
        /// Upload image
        ///</summary>
        [ImplementPropertyType("umbracoFile")]
        public Umbraco.Web.Models.ImageCropDataSet UmbracoFile
        {
            get { return this.GetPropertyValue<Umbraco.Web.Models.ImageCropDataSet>("umbracoFile"); }
        }

        ///<summary>
        /// Height
        ///</summary>
        [ImplementPropertyType("umbracoHeight")]
        public string UmbracoHeight
        {
            get { return this.GetPropertyValue<string>("umbracoHeight"); }
        }

        ///<summary>
        /// Width
        ///</summary>
        [ImplementPropertyType("umbracoWidth")]
        public string UmbracoWidth
        {
            get { return this.GetPropertyValue<string>("umbracoWidth"); }
        }
    }
}