RTC has a set of validation options, some built-in, some semi-built-in, and some you have to write.

RTC has several "built-in" which can be added as Preconditions using Operational Behaviors (Under Team Configuration).  These include marking something as required.  Make sure you also include "Attribute Validation" or it will only show errors and not block saving.

Next, look under Project Configuration -> Work Items -> Attribute Customization.  There is an option for Validators and there are a set of built-in providers:  Decimal Range, Number Range, Regular Expression, and Script-based.  For the APAR #, you could use Regular Expression and validiate it that way.  Script-based allows you to write a javascript which can use a set of javascript APIs to look at the values of the attributes and specify and error if the script determines a problem.

After that, you can go down to a Server-Side plugin which can do validation, which are written in Java and would be used if you the validation is more complex than what can be done in the first 2.

Susan