really not a very clear statement, but it SOUNDS like it's trying to make a distinction between a more descriptive and probably more detailed drawing and a diagram as a simpler analytical tool which edits out the detail toward communication of an idea, concept, parti, etc.

a diagram tends to have a specific agenda - a single idea or relationship between things - that it's trying to convey.

like i said, though, the statement doesn't make much sense. seems to me we still use both detailed drawings (er...renderings. hello.) and diagrams in communicating design ideas.

But I think something are there, that I am not sure aout it. For example I think there's a relationship between the difference between drawing and diagrams and thinking of management of architecural proccess or exactly,new way of thinking about all.

There's a distinction there that's common to the art world:

Idea vs. Technique - diagram is more about idea, drawing more about technique. In the past century, the majority of contemporary art has been about idea (with many great exceptions) - where technique has been primarily used to make the idea clearer, but not the primary subject of interest.

Art of antiquity, for example Renaissance art (and architecture) had perhaps more focus on technique - the idea behind the art existed, but was somehow bound by the technical norms of the day - Beaux Artes, etc. This makes me think about decoration when it has existed as a reiteration of the major idea behind a building (i.e. Rennaisance buildings where the capitals, cornices, etc. maintain idealized proportions relevant to the human body - the "ideal" of that period) - in other words, technique / details expressing overall idea.

I'm generalizing, but these are interesting distinctions to make - there's probably room to explore between the two poles. If you were to go after drawing unto itself as an iterative process, you might look into the thoughts of some of the minimalists of the mid 1900's - working to take out all meaning and reference other than the media itself.

What would a diagram about drawing look like?

but a diagram is always one drawing (of an observed phenomenon) and a drawing is always one or more diagrams confounded within that drawing. rather than posit an anterior opposition betwen both modii, i think it would be more helpful to see a drawing in terms of diagrammatic complexity - if you remove the outlines around the drawn shapes, the variations of dark and light patches is a diagram of light and its lack, if you remove the patches and keep the outlines you have the 'topographical' diagram of shapes in their contours.

"What would a diagram about drawing look like?"- Gabe Bergeron

the 'about' is rather vague in your question. nonetheless, a diagram (as well as a drawing) is always an indexical record of the activity of drawing. therefor, a diagram/drawing is always the indexical diagram of its drawing. since the drawing is already a complex state of the diagrammatic and since the diagram is a drawing, i would imagine that your answer would be : that very drawing itself.

the iconic vs symbolic view is interesting. a diagram is seen as symbolic because it needs to be interperated. the assumption is that one needs to be provided with layering signs that could decipher the signs dormant in the diagram. the drawing is an iconic representation that retains its umbilical cord to the phenomenal world. therefor, there is less of an intermediacy in deciphering the drawing. however i see this as too much of a forced creation of different natures of signs. the connoted distinction is between the abstracted (diagram) and the literal (drawing)...but the drawing is already an abstraction of the observed, as much as the observed is an abstraction of the environment observed. so again, i see it more as a complex and variant state of sameness rather than that of an interplay between two very different creatures.