  For relativistic modelling of high-accuracy astronomical data several time
scales are used: barycentric and geocentric coordinate times, TCB and TCG, as
well as two additional time scales, TDB and TT, that are defined as linear
functions of TCB and TCG, respectively.
  The paper is devoted to a concise but still detailed explanation of the
reasons and the implications of the relativistic scalings of astronomical
quantities induced by the time scales TDB and TT.
  We consequently distinguish between quantities and their numerical values
expressed in some units.
  It is argued that the scaled time scales, the scaled spatial coordinates and
the scaled masses should be considered as distinct quantities which themselves
can be expressed in any units, and not as numerical values of the same
quantities expressed in some different, non-SI units (``TDB units'' and ``TT
units'').
  Along the same lines of argumentation the system of astronomical units is
discussed in the relativistic framework. The whole freedom in the definitions
of the systems of astronomical units for TCB and TDB is demonstrated. A number
of possible ways to freeze the freedom are shown and discussed. It is argued
that in the future one should think about converting AU into a defined quantity
by fixing its value in SI meters.
