More info here
Sets the main LE window that will receive link editor commands and open the unofficial update link editor toolbar in a floating window.
Both commands need an active link editor window to run.
The A button will set the currently active link editor window as the window to receive link editor commands. By default trueSpace sends the commands to the first opened LE window.
The T button will open the Unofficial Update Link Editor toolbar in a floating toolbar in the upper left of the active link editor window.