public void setRegions(java.util.Collection<String> regions) {
        if (regions == null) {
            this.regions = null;
            return;
        }

        this.regions = new com.amazonaws.internal.SdkInternalList<String>(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OUTDATED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>

@param regions
       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OUTDATED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