Conversion of expressions by the FrontEnd

Question

As I learned recently there are some types of expressions in Mathematica which are automatically parsed by the FrontEnd.

For example if we evaluate Hol

Answer 1

There are two aspects to what you witness. First, transcription of the expression you entered into boxes and rendering those boxes by Front-End. By default the output is typeset using StandardForm, which has a typesetting rule to render graphics and geometric transforms. If you use InputForm, there are no such rule. You can control which form is used via Preferences->Evaluation.

You can convince yourself that HoldComplete correctly did its job by using InputForm or FullForm on the input, or using InputForm display on the output cell.

EDIT Using the OutputForm:

In[13]:= OutputForm[%]

Out[13]//OutputForm= HoldComplete[Rotate[expr, 0.5]]

In regard to your question about complete list of symbols, it includes Graphics, geometric operations, and possibly others, but I do not know of the complete list.