Orchard CMS (2) Meaning of some code in the migrations class

Example:

public int Create() {            // Create the ActionLinkPartRecord table            SchemaBuilder.CreateTable("ActionLinkPartRecord", table => table                .ContentPartRecord()                .Column<string>("ActionName", c => c.WithLength(256))                .Column<string>("ControllerName", c => c.WithLength(256))                .Column<string>("AreaName", c => c.WithLength(256))                .Column<string>("RouteValues", c => c.WithLength(256)));            // Define the ActionLinkPart            ContentDefinitionManager.AlterPartDefinition("ActionLinkPart", part => part                .Attachable(false));            // Define the ActionLink content type and set it up to turn it into a menu item type            ContentDefinitionManager.AlterTypeDefinition("ActionLink", type => type                .WithPart("ActionLinkPart")     // Our custom part that will hold the Action, Controller, Area and RouteValues information                .WithPart("MenuPart")           // Required so that the Navigation system can attach our custom menu items to a menu                .WithPart("CommonPart")         // Required, contains common informatin such as the owner and creation date of our type. Many modules depend on this part being present                .WithPart("IdentityPart")       // To support import / export, our type needs an identity since we won;t be providing one ourselves                .DisplayedAs("Action Link")     // Specify the name to be displayed to the admin user                // The value of the Description setting will be shown in the Navigation section where our custom menu item type will appear                .WithSetting("Description", "Represents a custom link with a text and an action, controller and routevalues.")                // Required by the Navigation module                .WithSetting("Stereotype", "MenuItem")                // We don't want our menu items to be draftable                .Draftable(false)                // We don't want the user to be able to create new ActionLink items outside of the context of a menu                .Creatable(false)                );            return 1;        }