When setting up your Pro X and XS units, you're asked to select the number of expansion units.
This may be a bit confusing, but is actually quite simple.
When setting the Pro X, the unit is asking how many XS units are in line before the Pro X.
'No XS' would mean that it is the first QCon in line, to the far left. If you're using extenders, they're assumed to follow to the right.
'One XS', means that there is one XS to the left of the Pro X unit.
In the case of the XS however, 'One XS' means that it is the first unit in line to the left, not that there is one before it.