What's the difference between the two-column layout and two-column bricks layout? I enabled the experimental Field Layout core module. There are two different two-column layouts available: one called "two column" and another called "two column bricks."

The dialog provides the diagrams below which don't clue me into their meaning. What is the difference between these layouts?
Layouts are defined in the Layout Discovery module and can be used by different layout modules.

The difference between those two layouts is the number of regions,  the Two Column Brick layout has three more. See the yaml layout definition:

/core/modules/layout_discovery/layout_discovery.layouts.yml

layout_twocol:
  label: 'Two column'
  path: layouts/twocol
  template: layout--twocol
  library: layout_discovery/twocol
  category: 'Columns: 2'
  default_region: first
  icon_map:
    - [top]
    - [first, second]
    - [bottom]
  regions:
    top:
      label: Top
    first:
      label: First
    second:
      label: Second
    bottom:
      label: Bottom

layout_twocol_bricks:
  label: 'Two column bricks'
  path: layouts/twocol_bricks
  template: layout--twocol-bricks
  library: layout_discovery/twocol_bricks
  category: 'Columns: 2'
  default_region: middle
  icon_map:
    - [top]
    - [first_above, second_above]
    - [middle]
    - [first_below, second_below]
    - [bottom]
  regions:
    top:
      label: Top
    first_above:
      label: 'First above'
    second_above:
      label: 'Second above'
    middle:
      label: Middle
    first_below:
      label: 'First below'
    second_below:
      label: 'Second below'
    bottom:
      label: Bottom