I’m going to start this definition by setting an example
which will then take us to Crary’s use of the word so that we can understand the
meaning of ‘assemblage’ inside the field of the techniques of the observer.

A textbook represents an assemblage because it has both a use
and a connotation inside its meaning. A textbook is perceived as an
epistemological object (something that you can use to get knowledge) and at
the same time as a stereotypical component of education. One can’t look at a
textbook and don’t think about the social things that it represents: school,
education system, student, subject, etc.

This is precisely what Crary argues about the camera obscura
(and potentially every optical device): “its ‘mixed’ status as an epistemological
figure within a discursive order and
an object within an arrangement of cultural practices”.

Now we can introduce the sentence that Crary uses to define
an assemblage in words that would be difficult to understand without the
examples. I’ going to quote this sentence pretending it’s a definition from the
dictionary. Assemblage: “an object about which something is said and at the
same time an object that is used”.