absdelay and changing td

A user here was trying to use the absdelay() function
and I am now trying to make sense of what the AMS LRM
has to say about absdelay() in Section 4.4.8:

  absdelay() implements the absolute transport delay for 
  continuous waveforms (use the transition operator to 
  delay discrete-valued waveforms). The general form is
      absdelay( input, td [, maxdelay ] )
  input is delayed by the amount td. In all cases td shall
  be a positive number. If the optional maxdelay is specified,
  then td can vary; but it shall be an error if it becomes
  larger than maxdelay. If maxdelay is not specified, changes
  to td shall be ignored. If maxdelay is specified, changes to
  td are ignored and the initial value of maxdelay is used.

Please focus on the last two sentences.  It appears to me
that changes to "td" are *always* ignored (except that if
td > maxdelay, you get an error).

Can someone please explain the intent?  Why not have "td"
be a constant expression and dispense with "maxdelay"?

"the initial value of maxdelay" is odd wording, also, since
maxdelay is required to be a constant expression.

-Geoffrey