There isn't much that's publicly available regarding the underlying implementation of those classes/methods. Here are some resources though that might help:
Maximo BIRT Report Developer Guide. Sections 6 and 7 of this document contain a lot of helpful references to classes and methods that can perform useful functions in BIRT Reports for Maximo. They even detail some methods available on the classes you list:
https://www.ibm.com/support/pages/system/files/inline-files/$FILE/Maximo76_Designer431_Report%20Development%20Guide_Rev8.pdf
BIRT Built-in Javascript functions. This reference guide details classes and method signatures for helpful Javascript functions that can be called within Maximo BIRT Reports. I tend to use the BirtDateTime and BirtStr classes the most:
https://www.ibm.com/docs/en/radfws/9.7?topic=reference-class-birtdatetime#259860
Hope this helps,
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Alex Walter
A3J Group LLC
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