PARENT + CHILD HEIGHT
A good example of Parent Frames is when you have a Column or a Grid with Text on Top of a Background Image. In this case you'll want to have a Parent and Child relationship:
PARENT: The Parent Frame has a Fixed Height with a Background Image.
CHILD: The Child Frame includes Text. You can control the Height of the Child Frame with the Child Height. Consider giving the Frame and Background Colour or Overlay.
Since the Child Height is a % height, the Parent must always have known Fixed Height, this minimum Height can not be used.
By Default, when you add a Child Height, this will be set to Desktop Only, meaning it will not apply to Mobile. This is the default behaviour since a Mobile is a smaller screen than Desktop, and therefore the Child elements most often will fill the screen, and therefore no height is required.
However in some cases, the Child elements on Mobile do not fill the screen, e.g. Title Bars. In which case, you can set Fixed Height Devices to 'All Devices' to ensure you Height will also display.