Layout Group Class
Structure If this is the topmost layout group in a window, a ListTree editor can be used to modify the hierarchy interactively.
Format If set to Horizontal, the children of this group will be arranged in a horizontal group. Similar for Vertical. Rows and Columns will create an array with the indicated number of rows or columns.

A horizontal group:

A vertical group:

A group with Rows=2 or Columns=3:

Rows
Columns
The row or column count, for the appropriate Format type.
Spacing If set to Default, the MUI preferences group spacing is used. Otherwise, the indicated number of pixels are used between objects.
Horiz Spacing
Vert Spacing
The spacing, in pixels, between objects for spacing type of Custom

. A group with Horiz Spacing=10: