additional capabilities of a Panel
https://docs.sencha.com/extjs/7.0.0/...omponents.html

probably refers to
The Panel class has many capabilities:

  • Border
  • Header
  • Header tools
  • Footer
  • Footer buttons
  • Top toolbar
  • Bottom toolbar
  • Containing and managing child Components

If the required UI Component is to contain other Components, but does not need any of the previously mentioned additional capabilities of a Panel, then Ext.container.Container is the appropriate class to extend.

This Implies
Ext.container.Container does not contain 'Containing and managing child Components', if that is an 'additional capabilities of a Panel,'.