When is a Message Base not a Message Base

Summary

trying to understand where this meta data is pulled from

Content

Apologies for this fairly simple query but i cant find the answer in the documents

In the customer portal pages the tab titles when the page is displayed say "customer service", and when i look at the source in the template file it shows

<title><rn:page_title /></title>

i thought this was a message base but after a bit of looking in the docs its says the naming convention is

rn:msg#...#

Can anyone point me in the right place to make this change rather than me just hard coding the required page title