The meaning of the css classes provided by IBM to create custom layout in IBM Portal Theme ?

The meaning of the elements is as follows:

class="component-container"

The static page parser recognizes the microformat to define containers in the page layout model that can contain components. Components can be portlets or widgets.

name="ibmMainContainer"

The name attribute on a component container identifies that container uniquely to the page. The static page parser uses the name to correlate containers when it updates the page definition. Using consistent names across layout templates preserves your container contents when you switch between layouts on pages. There must always be a container that is named ibmMainContainer for the main content of the page.

name="ibmHiddenWidgets"

When you create your own layouts, there must always be the one container at the beginning of the page named ibmHiddenWidgets.

name="headline"

If there are containers other than ibmHiddenWidgets or ibmMainContainer, use headline for a header or banner across the page for smooth switching between your layouts and the default layouts.

name="secondary"

Use secondary for content that is secondary to the main content, such as a sidebar.

name="tertiary"

Use tertiary for content that supports the secondary content.

name="additional"

Use additional for content that supports the tertiary content.

name="footer"

Use footer for an item at the end of the page.

class="ibmDndColumn"

class="ibmDndRow"

This class is required for client-side drag-and-drop support. Mark your component container with one of these classes.

class="wpthemeCol"

class="wpthemeRow"

This class provides CSS positioning information. Mark your component container with one of these classes.

class="wpthemeHeadlineContainer"

class="wpthemePrimaryContainer"

class="wpthemeSecondaryContainer"

class="wpthemeTertiaryContainer"

These classes provide CSS sizing information. Mark your component container with one of these classes. These classes are optional. You can use one of your own classes if your container sizing requirements vary from what these provide.

class="wpthemeLeft"

This class provides CSS positioning information. Mark your component container with this class if it is one of multiple containers on the same vertical level and floats to stay on the same level.

https://www.ibm.com/support/knowledgecenter/SSFTN5_8.0.1/com.ibm.wbpm.admin.doc/customizing/cadm_bsp_custompagelayout.html