The default TimeScale value depends on the current SystemSystem value; if the current TimeFrame system is "Besselian epoch" the default is "TT", otherwise it is "TAI". Note, if the System attribute is set so that the TimeFrame represents Besselian EpochEpoch, then an error will be reported if an attempt is made to set the TimeScale to anything other than TT.
Note, the supported time scales fall into two groups. The first group containing UT1, GMST, LAST and LMST define time in terms of the orientation of the earth. The second group (containing all the remaining time scales) define time in terms of an atomic process. Since the rate of rotation of the earth varies in an unpredictable way, conversion between two timescales in different groups relies on a value being supplied for the Dut1Dut1 attribute (defined by the parent FrameFrame class). This attribute specifies the difference between the UT1 and UTC time scales, in seconds, and defaults to zero. See the documentation for the Dut1 attribute for further details.