A Codegroup is a field-container and is available across all callsystems of a Business Partner.
CodeGroups can store various Codes (Callstates, Priorities, Severities, FailureTypes, ProblemTypes, Urgencies, Impacts)) in ServiceGrid.
They're very usefull if you need to create Calllists or Cockpit Reports of Tickets across several Callsystems as a ServiceGrid implementation specialist. Codegroups help SG customers to monitor/handle and even use the reporting function for tickets across different workflows.
Our new Feature "Active SLA" is using Codegroups for prioritys as a filter for the Radarscreen too (Filter option "Priority", see http://docwiki.cisco.com/wiki/ServiceGrid_Article_-_Active_SLA_Radar).
Hope that helps!
Thanks & regards,
Alex