What is a “Master Source” on Source Description page?

You can have more than one Source defined on the “Source Description” tab. One of those should be defined as the “Master Source”.

If you have only one source, it is the master (even if not marked). If you have more than one source, you must have a join in order to use the second source. All non-master sources must be connected with a join. If a source, other than the master does not have a join, it won’t be used.

The master source is the source that the import process runs through. Join tables behave as lookups off the master source (or other parent source). The lookup may fail and return null when the child doesn’t have a corresponding row. But, for each parent key in the master, the process expects to find just one unique row in the child table.

One of the most common uses of the join is to limit the parts in a segment. It’s a common mistake, however, for users to try to use a list of parts as the master; this doesn’t work.

Often the child source would have multiple rows for each part. In this case, the lookup fails because there isn’t a unique row in the child table.

The way to limit by parts is to make the table with the data for the segment the master and the list of parts the child. Then check “Remove null rows” for the join. This removes rows from the master where the part doesn’t exist in the part source.

Each segment is completely independent of the other segments.