A portal is a rectangle on a certain plane, it define a rectangular  region  for a specific plane. Portal can work with Section  for Visibility Culling; also can be a mirror; or can be used for Render To Texture.
To create portal in Awakening, click the drop down arrow right at in main toolbar, select 'Portal' in the popup menu, then drag on a Surface object.

Name Name of portal
Disabled Disable portal for Visibility Culling, but not for Render To Texture.
Mirror Indicates portal is a mirror
Flip Flip portal
Clip Offset for RTT The clip plane offset for Render To Texture
Render To Texture Render scene to texture,  cut out scene in front of portal (portal self is the clip plane, will be offsetted by the forenamed parameter).
Render To Texture (Mirror) Render mirrored scene to texture. (doesn't care portal's Mirror style)