Gear Master
When an axis is assigned as the master, its trajectory generator output (not its feedback position) is used as the master position command. This produces synchronized movements without taking the master's following error into account. This mode of operation can eliminate the following error skew between the master and slave axes and is especially useful in gantry applications. The master axis can operate in any mode (including being a slave to another master).
Selecting Encoder sets the axis as a slave to the master's encoder. This produces a following affect and takes all following error jitter and behavior of the master into account.
Selecting ADC is similar to selecting Encoder, but assumes that the master is using ADC feedback rather than encoder feedback.