Questions about property resolution

[Cédric Teyton]

Hello,

I've a question with respect to some POM.xml files i've observed 
recently. In some cases like the following one :

<project>
<modelVersion>4.0.0</modelVersion>
<groupId>activemq-web</groupId>
<artifactId>activemq-web</artifactId>
<name>ActiveMQ :: Web</name>
<version>4.0-M2</version>
<description>Web Connector for REST API and Streamlets support</description>
<dependencies>
    ...
<dependency>
<groupId>springframework</groupId>
<artifactId>spring</artifactId>
<version>${spring_version}</version>
</dependency>
    ...
</dependencies>
</project>

I just don't figure out how the variable /${spring_version}/ can be 
matched to something.
Indeed, there's no property specified in this POM, and there is also no 
parent.

Have you an idea about how this property can find an actual value ?
Thanks a lot,

Cheers
Cédric