How are you,
The open api is the documentation for the rest apis (One of the tools to implement is swagger).
The open apis of TMforum is the specifications or standard proposed by the TMForum for apis related to telco business.
If you want to start , try to this link
------ Development Course ------
https://projects.tmforum.org/wiki/display/API/Developer+Quick+Start+GuideI hope it helps you