gnucash/Docs/xacc-groups.html

<h1>Account Groups</h1>

Typically accounts are arranged in a tree.  Typically,
Accounts that are not leaf accounts have a number that ends in zero.
For example:

<pre>
        300             Expenses
         |
         +--310         Living Expenses
         |   |
         |   +--311     Beer
         |   |
         |   +--312     Cable
         |
         +--320         Business Expenses
         |   |
         |   +--321     8-inch Floppies
         |   |
         :   :


</pre>