each section of the process config has a checkbox called Final.

if that checkbox is on, then children projects cannot modify that section of the process config.

note that in most cases this is a very big area..

All types & attributes, all enumerations, all workflows.

once a child project modifies a section WITHOUT final checked,  that section for them becomes a copy,
and any changes done later in the parent are not seen by the child.
you CAN set final later, and immediately all children that modified  a section will immediately LOSE any modifications they have done. it is NOT a merge.. its either parent or local.

workitems created WHILE changes are enabled, will be weird if final is turned on later.  you cannot delete attributes, or workitem states.. For example, you add a tab and some attributes as part of a presentation, then the parent disables the ability to do that.. poof the layout disappears and the variables disappear on the web, and move to the custom tab in Eclipse.

one of the more intriguing things to debug is the final flag effect. cause child projects are still allowed to EDIT the process area in many cases.. and SAVE their changes..  but nothing happens!.. its really a strange effect.. cause there have been bugs in the past that behave the same way,.

now, for limited changes.. enums, attributes and presentations only, there is support for local project support, if you use the right config setup. 
https://jazz.net/wiki/bin/view/Main/ConfigurationDataDeltaUserDoc
and
https://jazz.net/forum/questions/167461/rtc-process-sharing-design-considerations-for-consumer-project-areas