The number of teeth on the spindle gear.
The rotary encoder produces a fixed number of electrical pulses for each turn of its shaft. The shaft of the encoder has to be connected to the lathe spindle which drives the chuck. This is usually done by putting a gear on the shaft of the encoder (the encoder gear) to mesh with the gear on the spindle (spindle gear) or the stud gear. If the encoder gear does not have the same number of teeth as the spindle gear, this gear ratio has to be taken into account.

Teeth on the encoder gear

The number of pulses produced for every revolution of the encoder shaft.
This number typically ranges from 200 to 4096 p/r.

If the lead-screw is metric the lathe is defined as a metric lathe.

Pitch or TPI of the leadscrew.
eg 8 TPI or 3mm pitch (do not enter units).

Thread type.

Required thread TPI or pitch (mm).

GTR: Enter 1 if you do not have a leadscrew gearbox. GTR is the gear ratio of the gear train required to cut a thread the same as the leadscrew.

If you have a leadscrew gearbox you may not need ELS, but if you do, we have to take into account the gear ratio introduced by the gearbox. Elsewhere in RideTheGearTrain I call this the 'Primary Ratio' of the gearbox.
Set up your gearbox and gear train to cut the same pitch or TPI as the leadscrew. Make note of the gearbox setting and use this in future when you use the ELS. Now look at this 'standard gear train' and work out its gear ratio or 'GTR'. Typicaly, this consists of a stud gear and a leadscrew gear (LSG) separated be a larger idler gear. The idler is just used to fill the gap and does not affect the gear ratio (see help). The stud gear is the driver gear and goes on the top line of the gear ratio, while the LSG is driven and goes on the bottom line. So, divide the number of teeth on the stud gear by the LSG, and this is the GTR which you enter here. I will use my imperial Boxford A lathe as an example: The leadscrew is 8 TPI. If we set it up to cut a thread with 8TPI we know the overall gear ratio must be 1. However, the standard setup uses a gear train with the stud gear having 20 teeth and the LSG has 56 teeth giving GTR=20/56=1/2.8. This means that the gearbox must be producing a gear ratio of 2.8 so that the overall gear ratio of gear train and gearbox is 1.0, ie 2.8 x 1/2.8 =1.

The number of steps per revolution of the motor shaft
This number is typically 24, 48 or 200 (sometimes 400 or more) s/r.

Some motors divide each step into a number of microsteps. If you do not use microsteps, enter 1
e.g. Some motors provide 24 steps with 1024 microsteps giving a total of 24,576 microsteps per revolution. You do not really need that degree of resolution to drive a leadscrew, and there are tradeoffs in reduced torque and maximum RPM.

Teeth on the motor gear
This gear is mounted on the motor shaft and meshes with the leadscrew gear to drive the leadscrew.

The number of teeth on the leadscrew gear.
This is the gear on the leadscrew shaft which is used to connect the stepper motor.