I checked the Table SEM_Agent from the schema guide, it does not say anything about what it means, its blank with no comments
https://support.symantec.com/en_US/article.DOC8633.html
I belive that its the IP ( in case of dual IP) which is used to connect to SEPM
or
SEPM IP ( In case of multiple SEPMs in case of fail over)
it might be symantec internal better to have a case created to get it clarified, when you do , please dont forget to update this thread.