An autotransformer contains only one coil, so the primary and secondary sides always have just one conductor between them. The primary and secondary side voltages of an autotransformer do not have galvanic isolation.
An autotransformer can be built with a significantly smaller frame, if the primary voltage (U1) and the secondary voltage (U2) values are similar.
S2 = Sn x (1-Un / U1)
S2 = secondary side load (VA)
Sn = transformer’s nominal power (VA)
U1 = upper voltage (V)
U2 = lower voltage (V)