Doubt on <bean> tag

[sudhakarbe]

<bean id="tstbean">
<property name="suffix" value=".html"/>
<property name="templateUrls">
<props>
<prop key="/plan">${templates_tariffplan}</prop>
</props>
</bean>

what is '${templates_tariffplan}'?
how is this value getting replaced in 'flow-servlet.xml'