It defines a set of custom attributes. Custom attributes are objects associated with a particular scope. Acceptable scopes include component, space, page, desktop, session and application.

As depicted below, custom-attributes is convenient to assign custom attributes without programming.

<window>
    <custom-attributes main.rich="simple" very-simple="intuitive"/>    
</window>

It is equivalent to

<window>
    <zscript>    
        self.setAttribute("main.rich", "simple");        
        self.setAttribute("very-simple", "intuitive");        
    </zscript>    
</window>

Moreover, you could specify what scope to assign the custom attributes to.

<window id="main" title="Welcome">
    <custom-attributes scope="desktop" shared="${main.title}"/>    
</window>

It is equivalent to

<window id="main">
    <zscript>    
        desktop.setAttribute("shared", main.title);        
    </zscript>    
</window>

Notice that EL expression is evaluated against the component being created. Sometime it is subtle to notice. For example, ${componentScope.simple} is evaluated to null, in the following codes. Why? It is a shortcut of <label value="${componentScope.simple}"/>. In other words, the component, self, is the label rather than the window, when the EL is evaluated.

<window>
    <custom-attributes simple="intuitive"/>    
    ${componentScope.simple}    
</window>

is equivalent to

<window>
    <custom-attributes simple="intuitive"/>    
    <label value="${componentScope.simple}"/><!-- self is label not window -->    
</window>

Tip: Don't confuse <attribute> with <custom-attributes>. They are irrelevant. The attribute element is a way to define a XML attribute of the enclosing element, while the custom-attributes element is used to assign custom attributes to particular scopes.