if it is a "web application" project - use CodeBehind
if it is a "website" project - use CodeFile
in regular ascx file:
<%@ Control Language="C#" AutoEventWireup="true" Inherits="My.Great.Web.SomeCtrl" CodeBehind=SomeCtrl.ascx.cs" %>in regular aspx file:
<%@ Page Language="C#" AutoEventWireup="true" Inherits="My.Great.Web.SomePage" Codebehind="SomePage.aspx.cs" %>
in global.asax:
<%@ Application Language="C#" Inherits="My.Great.Web.Global" CodeBehind="Global.asax.cs" %>
More info here: ASP.NET Web Page Code Model
