Z indicates the gear you install from row Z on the chart to give you the different thread pitches listed under the gear numbers specified in row Z.
For example, the lathe ships stock with the 24T on top & the 48T on bottom, both meshed to the 127T. So in this configuration you are able to cut the follow thread pitches: 16, 32, 18, 36, 20, 40, 24, 48, 28, & 56 just by changing dials.
All the other thread pitches you'd have to change gears around. And for metric threads, the bottom gear has to mesh with the 120T instead of the 127T, The top gear will always mesh with the 127T.