Validating System Configuration Values

<!-- # Module Initialisation
    app/etc/modules/Namespace_ModuleName.xml -->

<config>
    <modules>
        <Namespace_ModuleName>
            <active>true</active>
            <codePool>local</codePool>
        <...

Frontend:

<config>
    <frontend>
        <layout>
            <updates>
                <namespace_module>
                    <file>namespace-module.xml</file>
                </namespace_module>
            </up...

<config>
    <global>
        <models>
            <namespace_module>
                <class>Namespace_Module_Model</class>
                <resourceModel>namespace_module_resource</resourceModel>
            </namespac...

<config>
    <global>
        <blocks>
            <namespace_module>
                <class>Namespace_Module_Block</class>
            </namespace_module>
        </blocks>
    </global>
</config>

![](http://m...

<config>
    <crontab>
        <jobs>
            <cron_job_name>
                <schedule>
                    <cron_expr>* * * * *</cron_expr>
                </schedule>                
                <run>

...

EAV:

<config>
    <global>
        <resources>
            <namespace_module_setup>
                <setup>
                    <module>Namespace_Module</module>
                    <class>Mage_Eav_Model_Entity_Setup</...

Frontend:

<config>
    <frontend>
        <events>
            <the_name_of_the_event_to_observe>
                <observers>
                    <namespace_module>
                        <class>namespace_module/obser...

Frontend With Route:

<config>
    <frontend>
        <routers>
            <namespace_module>
                <use>standard</use>
                <args>
                    <module>Namespace_Module</module>
       ...

The decision of whether to show or hide a product based on this setting is implemented
by the Product Price Index.
You can verify this by changing the setting to "Yes" and then viewing ...

Same for 1 and 2