if you're talking about SQL Agent jobs, they correspond to logins (sysjobs has a sid column as a result). Since roles (at least ones we can define) are at the database level, they can't be assigned as the owner of a job.
K. Brian Kelley, GSEC
http://www.truthsolutions.com/
Author: Start to Finish Guide to SQL Server Performance Monitoring
K. Brian Kelley
@kbriankelley