The below screenshots shows how you can combine 2 data sets to create a single data set containing all records from both. (i.e. the equivalent of a union in SQL). The same principal applies if you are also combining more than 2 streams.
If you refer to an attribute without a pipe name prefix, i.e. Attribute1, rather than in1.Attribute1, then PhixFlow expects that attribute to exist in all source pipes.
If you do not put an attribute expression, then PhixFlow expects to find an attribute with exactly the same name as itself in all source pipes.
If your attribute is called something different in each source stream, then you will need to check the pipe name to work out which attribute to use. This is shown in Attribute3 in the screenshot. _pipeName is an internal variable that stores the pipe name of the current record.
Further explanations/ examples can be found here: /wiki/spaces/CONFIGKB/pages/97734760