SelectCheckboxMenu SelectManyCheckbox is used to choose multiple items displayed in an overlay.
    Selected Cities
    Basic
    No cities selected
    Multiple
    No cities selected
    <h:form>
        <h:panelGrid columns="2" cellpadding="5">
            <h:outputLabel for="menu" value="Basic:" />
            <p:selectCheckboxMenu id="menu" value="#{checkboxView.selectedCities}" label="Cities" 
                                  filter="true" filterMatchMode="startsWith" panelStyle="width:250px">
                <f:selectItems value="#{checkboxView.cities}" />
            </p:selectCheckboxMenu>
    
            <h:outputLabel for="multiple" value="Multiple:" />
            <p:selectCheckboxMenu id="multiple" value="#{checkboxView.selectedCities2}" label="Cities" multiple="true" 
                                  filter="true" filterMatchMode="startsWith" panelStyle="width:250px">
                <f:selectItems value="#{checkboxView.cities}" />
            </p:selectCheckboxMenu>
        </h:panelGrid>
    
        <p:commandButton value="Submit" update="displayCities" oncomplete="PF('cityDialog').show()" style="margin-top:10px;" />
    
        <p:dialog header="Selected Cities" modal="true" showEffect="fade" hideEffect="fade" widgetVar="cityDialog" width="250">
            <p:outputPanel id="displayCities">
                <p:dataList value="#{checkboxView.selectedCities}" var="city" emptyMessage="No cities selected" style="margin-bottom: 10px;">
                    <f:facet name="header">
                        Basic
                    </f:facet>
                    #{city}
                </p:dataList>
    
                <p:dataList value="#{checkboxView.selectedCities2}" var="city" emptyMessage="No cities selected">
                    <f:facet name="header">
                        Multiple
                    </f:facet>
                    #{city}
                </p:dataList>
            </p:outputPanel>
        </p:dialog>
    </h:form>
                    
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    Running PrimeFaces-6.0.16 on Mojarra-2.2.8.