Check out example codes for "Response.Redirect cannot be called in a Page callback". It will help you in understanding the concepts better.
Code Example 1
// Butchered together from multiple answers
if (!Page.IsCallback)
Request.Redirect("~/Error.aspx");
else
HttpContext.Current.Response.RedirectLocation =
System.Web.VirtualPathUtility.ToAbsolute("~/Error.aspx");
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