The Chord Transposition tool is designed to help you transpose short chord progression!
By writing in the progression in the top input field and setting the key you get the output from the transposition in the lower input field!
If the string is C - G and you set the current key to be C and the "Transpose to" key to be G. It will transpose the progression to G - D.
It will have the same effect setting the current key to E and the "Transpose to" key to B. The software will not try to correct your set tonic.
In other words, transposing the progression, for example, an M3 up can be achieved by setting the interval between the two notes to a major third so, for instance, C- E or G-B or D-F# etc....