You can set the default container for a pane in your skin.
This is also known as "Pane Level skinning"

This means a module dropped into that pane automatically gets the container you have set to the default container in the skin.
A user can still overrule this through the module settings, it's just the default container for that pane.

There are 3 attributes you need to supply to set the default container for a pane.
All three are about the location of the container file to use.

Containertype
The container type sets if this is a Host or Site container.
For a Host Container you should set this attribute to "G" (global)
For a Site (Portal) Container you should set this attribute to "L" (local)

Containername
This is the folder the container is in (inside the "Containers" folder).
In general this resembles the Container Name

Containersrc
The actual container file you want to use inside the container package.
You should supply the filename of the ASCX container, not the HTML container.
(Example; "MyContainer.ascx")

An example in an ASCX skin would be like this:

<div id="Pane" containertype="L" containername="FolderName" containersrc="MyContainer.ascx" runat="server"> </div>

Please note that if you set the "ContainerType" to a Host container and you install the Container on a site as a Site/Portal container, you will get an error as in that case the container will not be found