The include and master server directives that avoids repetition of code in your files.

Inc­lude di­rec­tive(file)
This is used for including the contents of one or more files into a web page. This could commonly be a common piece of code throughout a site, such as a page header, a page footer and a navigation menu.
Mas­ter di­rec­tive(file)
This is used for specifying a master file to use as template for the page. By using XDT attributes you can swap, remove and insert content into the specificed master file.