The "relative" is calculated according to "parent". For example
local labelA = guiCreateLabel(100,100,400,200,"",false)
local labelB = guiCreateLabel(0.1,0.2,0.5,0.5,"",true,labelB)
for labelB, it's
x = 0.1 * 400
y = 0.2 * 200
width = 0.5 * 400
height = 0.5 * 200
In this way, we can know that:
x = relativeX * parentWidth
y = relativeY * parentHeight
width = relativeWidth * parentWidth
height = relativeHeight * parentHeight
If there's no parent, parentWidth and parentHeight will be screenWidth and screenHeight.