public java.util.List<String> getRegions() {
        if (regions == null) {
            regions = new com.amazonaws.internal.SdkInternalList<String>();
        }
        return regions;
    }
<p>
The Amazon Web Services Regions in which to update associated stack instances. If you specify Regions, you must
also specify accounts in which to update stack set instances.
</p>
<p>
To update <i>all</i> the stack instances associated with this stack set, do not specify the <code>Accounts</code>
or <code>Regions</code> properties.
</p>
<p>
If the stack set update includes changes to the template (that is, if the <code>TemplateBody</code> or
<code>TemplateURL</code> properties are specified), or the <code>Parameters</code> property, CloudFormation marks
all stack instances with a status of <code>OUTDATED</code> prior to updating the stack instances in the specified
accounts and Regions. If the stack set update does not include changes to the template or parameters,
CloudFormation updates the stack instances in the specified accounts and Regions, while leaving all other stack
instances with their existing stack instance status.
</p>

@return The Amazon Web Services Regions in which to update associated stack instances. If you specify Regions,
        you must also specify accounts in which to update stack set instances.</p>
        <p>
        To update <i>all</i> the stack instances associated with this stack set, do not specify the
        <code>Accounts</code> or <code>Regions</code> properties.
        </p>
        <p>
        If the stack set update includes changes to the template (that is, if the <code>TemplateBody</code> or
        <code>TemplateURL</code> properties are specified), or the <code>Parameters</code> property,
        CloudFormation marks all stack instances with a status of <code>OUTDATED</code> prior to updating the
        stack instances in the specified accounts and Regions. If the stack set update does not include changes
        to the template or parameters, CloudFormation updates the stack instances in the specified accounts and
        Regions, while leaving all other stack instances with their existing stack instance status.
aws-java-sdk-cloudformation/src/main/java/com/amazonaws/services/cloudformation/model/UpdateStackSetRequest.java