Hi Felicity,
To add something to Stuart's answer,
Backout are created automatically when copying any members from output library when the package is executed with option backout enabled.
When members are copied into the same library, Endevor then rename the old member with an encripted member name then copy the new member as is.
You have to take into consideration that the use of backout will double the number of members in your libraries (each member has a backout).
The package file will have pointers to the backout information as well as the new member.
If you start using backout and you reset, delete, or commit a package then the backout will be deleted.
If backout are not created for some elements, you need to check the processor used during package execution and check if the processor keyword BACKOUT=N has been set in the DD statement.
Let me know if this answer your question?
Regards,
Ollivier