A tool bar normally contain plain buttons. For example, the following plain button invokes native command convert in XMLmind XML Editor - Commands to convert the implicit or explicit selection to the bold element:

<button toolTip="Convert to bold" 
        icon="xxe-config:common/icons/boldText.png">
  <command name="convert"
    parameter="[implicitElement] {http://www.xmlmind.com/ns/sect}bold" />
</button>

However, we have chosen to add to the "Simple Section" tool bar text style toggles rather than plain buttons:

<button toolTip="Toggle bold" 
        icon="xxe-config:common/icons/bold.png">
  <class>com.xmlmind.xmleditapp.desktop.toolbar.TextStyleToggle</class>

  <command name="pass"
    parameter="{http://www.xmlmind.com/ns/sect}bold" />
</button>

Text style toggles emulate the behavior of the Bold, Italic, Underline, etc, toggles found in the tool bars of almost all word-processors. Such toggles are declared slightly differently than plain buttons. See Section 31.1.2, “The TextStyleToggle custom control”.