Unified Positions and Sizes System
From CEGUI Wiki - Crazy Eddie's GUI System (Open Source)
note: This system/page only applies to CEGUI >= 0.4.0
The system is explained the easiest way by examples:
<Property Name="UnifiedPosition" Value="{{0.1,10},{1.0,-30}}" />
X-position: 10% of the width of the parent window + 10 pixel Y-position: 30 pixel above the bottom of the parent window
<Property Name="UnifiedSize" Value="{{0.6,5},{0.3,20}}" />
Width: 60% of the width of the parent window + 5 pixel Height: 30% of the height of the parent window + 20 pixel
I hope you got the idea.