"A composer knows in advance where the loudest part of his composition is, and from there the available headroom, respectively the maximum loudness is set."
This shows a complete ignorance of performing and recording MIDI music. The loudest or softest notated or sequenced scoring might be far lower or higher in overall level depending on which samples are used and how they are processed.
"Level is recording technique terminology. For musical loudness we use the usual Italian terminology such a ppp or fff"
You still do not understand. A written ppp may sound higher in level than an fff, depending on how the sample is recorded and mixed. It is naive to assume writing a pp will have the slightest significance - except with timbral considerations - in the final mix. And yet recording and mixing have a huge influence on the musical expression.
Finally - if this is not about composing and orchestration then why did you put it on the composing and orchestration forum?