"Bounds": "x, y, x2, y2",
x & y, where this entity starts by the (top left?) corner I believe - x2 and y2 being the size of the image.
"Padding": "x, y, x2, y2",
I believe this one is self explanatory but it's the padding around the bounds.
"AlignmentEdgeDistances": "x, y, x2, y2", "AlignmentTransform": "x, y",
Don't worry about these unless your image just isn't going in the right spot (honestly if you're using this it might cause issues - stick to bounds)
"Margin": "x, y, x2, y2",
This one is mainly used for inventory/spells where you have an array of slots that need to be evenly spaced.
The way I work on my UI is estimate how many pixels my image needs to be and what each image within the whole needs to be. This allows me to use a calculator to precisely put everything where it needs to be.