CSC510249. Do not use SPWebPartManager when HTTPContext is null.
SharePoint supports a custom implementation of WebPartManager named SPWebPartManager. In addition, there is an SPLimitedWebPartManager class that supports environments that have no HttpContext or Page available.
SPWebPartManager its control that places to masterpage.
<WebPartPages:SPWebPartManager id="m" runat="Server"/>
SPLimitedWebPartManager its not control, it is specific class to work with functionality of web parts.
If you have HttpContext then use SPWebPartManager. When no HttpContext (in event receivers for example) is available you should use SPLimitedWebPartManager. It only supports a subset of features available in SPWebPartManager.