Import fake CDN, enable and sync three repositories. Five is recommended.

Create an environment "test" and create a changeset:

# kk  changeset create --name test3 --environment test

List all the repos:

# kk repo list
-----------------------------------------------------------------------------------------------
                    Repo List For Org ACME_Corporation Environment Library

 Id   Name                           Package Count  
-----------------------------------------------------------------------------------------------
 6    Zoo Enterprise x86_64 6.2      32             
 19   Zoo Enterprise x86_64 5.4      32             
 58   Zoo Enterprise i386 7          32             
 81   Nature Enterprise i386 1.0     32             
 84   Nature Enterprise x86_64 1.1   32  

Now list all the errata. In this case some repos has the very same errata IDs. In real life I believe those are always unique, but users could possibly create some with same IDs.

# kk errata list 
-----------------------------------------------------------------------------------------------
                                          Errata List

 Id               Title             Type      
-----------------------------------------------------------------------------------------------
 RHEA-2012:0002   Sea_Erratum       security  
 RHEA-2012:0003   Bird_Erratum      security  
 RHEA-2012:0001   Bear_Erratum      security  
 RHEA-2012:0004   Gorilla_Erratum   security  
 RHEA-2012:0002   Sea_Erratum       security  
 RHEA-2012:0003   Bird_Erratum      security  
 RHEA-2012:0001   Bear_Erratum      security  
 RHEA-2012:0004   Gorilla_Erratum   security  
 RHEA-2012:0002   Sea_Erratum       security  
 RHEA-2012:0003   Bird_Erratum      security  
 RHEA-2012:0001   Bear_Erratum      security  
 RHEA-2012:0004   Gorilla_Erratum   security  
 RHEA-2012:0002   Sea_Erratum       security  
 RHEA-2012:0003   Bird_Erratum      security  
 RHEA-2012:0001   Bear_Erratum      security  
 RHEA-2012:0004   Gorilla_Erratum   security  
 RHEA-2012:0002   Sea_Erratum       security  
 RHEA-2012:0003   Bird_Erratum      security  
 RHEA-2012:0001   Bear_Erratum      security  
 RHEA-2012:0004   Gorilla_Erratum   security 

Now let's add an erratum to the changeset. We need to add a product first:

# kk changeset update --name test3 --environment test --from_product "Zoo Enterprise" --add_repo "Zoo Enterprise i386 7"
Successfully updated changeset [ test3 ]

Then erratum. But from which repo: Let's try it:

# kk changeset update --name test3 --environment test --from_product "Zoo Enterprise" --add_erratum RHEA-2012:0001 
Validation failed: Repository of the erratum 'RHEA-2012:0001' has not been promoted into the target environment!

Oh wait, we dont know which one Katello picks up! Now you need to add ALL the repositories until you add the one that matches the erratum:

# kk changeset update --name test3 --environment test --from_product "Zoo Enterprise" --add_repo "Zoo Enterprise x86_64 5.4"
Successfully updated changeset [ test3 ]

This is LOW PRIO bug. We'd need to add some optioin like --from_repo maybe. Not sure.