how the content placeholder work?

In ASP.NET, the Master Page can contain a ContentPlaceHolder control that is used to define a region of the Master Page rendering that can be substituted with content from a page associated to the Master Page.

A content placeholder control is a piece of code on a master page that works together with a content control on a content page.
The content placeholder control displays default content (which can be no content, if the control is empty) for that region on the page but can be overridden by unique content from a content control on a content page. Any page using the master page can replace the content in a content placeholder control by supplying a matching content control. If a content control is empty, the content placeholder control gets overridden with an empty value that consequently removes the control from the page. see more...[^]

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