[over.best.ics.general] p6 says

When the parameter has a class type and the argument expression has the same type, the implicit conversion sequence is an identity conversion.

However, we specify the well-formed implicit conversion sequences in [over.best.ics.general] p3 as the following:

• a standard conversion sequence
• a user-defined conversion sequence
• an ellipsis conversion sequence

There is no identity conversion in the list. [over.best.ics.general] p8 says

If no conversions are required to match an argument to a parameter type, the implicit conversion sequence is the standard conversion sequence consisting of the identity conversion ([over.ics.scs]).

I think we should consistently use the wording in a similar manner to [over.best.ics.general] p8. That is, we should explicitly say what the kind of the implicit conversion sequence is and then specify what the conversions the sequence comprises. Such as:

When the parameter has a class type and the argument expression has a derived class type, the implicit conversion sequence is a standard conversion sequence consisting of the derived-to-base conversion from the derived class to the base class.