Let’s call “N-Way Sections”, the Multithreaded Sections that are executed using several CPU’s: Such sections are represented using the 
icon (instead of the “standard” 
icon). A “N-Way Section” ends with the 
Action and always starts with a 
Action.
Not all Actions can be included inside N-Way sections. For example, the 
Action or the 
Simple Join Action cannot be included inside a “N-Way Section”. Indeed, if we had the following (non-working) graph:
… This would be equivalent to the following “conceptual” graph:
The above illustration explains that each Sort Action process (i.e. sort) one quarter (¼) of the total number of rows. At this point, we only have a “partial” sort (and not a “complete” sort) because each sort was done on one quarter (¼) of the whole table (and not the whole table). Furthermore, the rows that are “going out” of the Sort Actions are put together, in a random order, inside the FIFO-Row-Buffer of the “second Multithread Action”. Thus, even the (partial) sorting of the row is lost (because of the random order of the rows inside the final FIFO-Row-Buffer on the right).