post contributed by Samit Mehta, IBM Rational Business Development]

Initial Development Environment Setup
You can start with the following to get your environment set up for development:
https://jazz.net/wiki/bin/view/Main/ComponentDevelopmentSetup

Depending on the type of extensions, you may also need to set up your Jazz Server for development/debugging server extensions. You can get details to set that up from the following:
https://jazz.net/wiki/bin/view/Main/JazzServerRunAndDebug

RTC 1.x.x.x SDK
There is no official/stable SDK for RTC. However, there are some customization capabilities. Also, there are some programmatic extension capabilities that are maintained to be stable at least for the 1.x.x.x release and may only change to address a significant issue. The details on the customization capabilities and the programmatic extensions are at:
https://jazz.net/wiki/bin/view/Main/RtcSdk

RTC Online Help
https://jazz.net/help/rational-team-concert/1.0.1/index.jsp

Process and Work Item customization
There are links to customizing the process and work items in the RTC SDK page. I presume that this is what you meant by "updating the XMLs". I am pasting these links below:

Process Customization (e.g. setting up out-of-the-box or custom preconditions, follow-up actions, and work item customization): https://jazz.net/wiki/bin/view/Main/ProcessSpecificationSyntax

However the easiest way to customize the process and work items are through the editors in the RTC client:
Work Item customization: https://jazz.net/learn/LearnItem.jsp?href=content/docs/work-items-cust/index.html
Process customization: https://jazz.net/help/rational-team-concert/1.0.1/topic/com.ibm.team.platform.doc/topics/t_custom_process.html

Programmatic access / customization of Work Items
As described in the RTC SDK, you can find some details on Programming interfaces for Work Items in the following locations:

Programmatic Work Item Creation: https://jazz.net/wiki/bin/view/Main/ProgrammaticWorkItemCreation
Programmatic Query of Work Items:
https://jazz.net/wiki/bin/view/Main/QueryDevGuide


Preconditions and Followup Actions
A very powerful capability to impact the process including process for Work Items is via preconditions and followup actions. You can find a lot of details around that here:
https://jazz.net/wiki/bin/view/Main/TeamProcessDeveloperGuide
https://jazz.net/wiki/bin/view/Main/PreconditionFollowupCreation
https://jazz.net/wiki/bin/view/Main/ProcessAspectEditorCreation

Deploying to the Jazz server
https://jazz.net/wiki/bin/view/Main/UsingJazzProvisioner