My auto transformer data sheet received from manufacturer has some parameter which i dunt understand.

It is

Short circuit impedance referred to the first winidng

At principal tap

High voltage -to low voltage 29.75 ohms per phase
Low voltage -to Tertiary 23.65 ohms per phase
High voltage -to Tertiary 99.6 ohms per phase

what does this mean?and whre can i use this parameter??