ASP.NET server-side comment blocks have the same uses as traditional language-specific comment blocks, including documentation and testing. For example, you can use server-side comments to describe sections of markup in a file, or to comment out one or more server controls declared in a page. These comments can be used in most types of ASP.NET application files, including Web pages, user controls, Global.asax files, master pages, and skin files
Note that server-side comments are to be used in the body of the page, but not within blocks of server-side code. When you are using a specific language within code declaration blocks (code that is contained within the <script runat="server"></script> tags) or code render blocks (code that is contained within <% %> tags), you should use the comment syntax for the language that you are coding in.
A compilation error will occur if you use server-side comment blocks within <% %> blocks.
The opening and closing comment tags can appear on the same line of code or can be separated by many commented-out lines.
Server-side comment blocks cannot be nested.