Resolution: Useful documentation is in Chapter 2 "Control Flow Patterns" and "Chapter 7 Working with Conditions" of "TIBCO InConcert Process Designer User’s Guide". The types of conditions which are used to test for the condition to branch on are the same type of condition used for perform and iterate conditions.
To set the task that it branches to you give the API the name of the task. Hence it is important that the names of tasks which immediately follow the decision task have distinct task names.
Nota Bene: You can have two or more children tasks of the Decision task. If the branch condition is true then the branch task becomes Ready and all other children task status becomes Excluded. When the branch condition is false then the branch task is Excluded and the other tasks become Ready.