Canvas Item Definitions

We see the use of images and text in canvas items in many examples available both from Roku and from other developers.
Is there a strict definition of all of the available combinations of the properties that a canvas item is made up from?

i.e. { url: "http://www.domain.com/mages/image1.png"
TargetRect: {x: borderPosX,
y: borderPosY,
w: 100,
h: 100}

This makes an image item.

What other combinations of properties will the Roku identify and create an item for? Is there a document that defines this somewhere?
Can the markup be more complex and have a container for items?

i.e. {list:[
{ url: "http://www.domain.com/mages/image1.png"
TargetRect: {x: borderPosX,
y: borderPosY,
w: 100,
h: 100},
{ url: "http://www.domain.com/mages/image2.png"
TargetRect: {x: borderPosX,
y: borderPosY,
w: 100,
h: 100},
{ url: "http://www.domain.com/mages/image3.png"
TargetRect: {x: borderPosX,
y: borderPosY,
w: 100,
h: 100}
]}


thank you,

Andrej