Basically incident master is a permissions and incident manager is functional role based in itil principles.
Ideally both more or less have the same job.
Incident master is a permission given to someone who is a master controller of incidents or situation manager. He/she has access to all the tickets at any point of time irrespective of the groups or status.
Incident manager is a functional role given to a person within a group who will be used as informational purpose or for sla notifications
For more information refer IT service management configuration guide
I recommend you check out the following tool
This is a graphic tool that walks you through what you want the user to do and tells you what permissions the user will need....
Incident Master permission grants access to the Incident Module with additional privileges to modify Incidents independent of any Functional Roles, or Support group affiliations. This permission supersedes Incident User, Incident Submitter and Incident Viewer.
Incident Manager is similar to Incident Master. Incident Manager is a functional role and it is specific to a Support Group whereas Incident Master is a Permission.
Incident Manager is a functional role and having functionality much similar to Incident Master.
Incident Master is a permission (independent of any functional role) grants access to the Incident Module with additional privileges to modify Incidents. This permission supersedes Incident User, Incident Submitter and Incident Viewer.
for more information , refer BMC Docs - Incident Management User roles.
LJ - I receive the following error when trying to access the tool you've provided...do you have another link?
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I'm also getting the same error....not sure what happened to it...