I personally think all the answers are valid and correct but ultimately Discrete Mathematics is full of 'Diagramatic data manipulation/permutation' and 'methodology'. (Unique functions and modelment)

What I mean simply is, you will find DM is full of 'Logic' and importantly 'Matrice' study or 'algorithms' (within Mathematics BSc to a very complicated degree).

It is these areas of mathematics that particularly operate with the sets of numbers, operations and functions 'clearly' Real {R,Z,Q and more} or 'generally' Unitised {R,Z,Q again}. Which is just the latin for the fact that as mentioned it is centred in fact around either discrete or continuous data which the RZQ sets are predominantly related to in element.

Its with this that I generalise the area down to modelment and logic, where of course all operations are either discrete or continuous as all say, but more particularly the most advanced areas of the subject operate near mostly with RZQ, (or Integers, the real set, all above 0, rational etc) That is the technical explanation for the opinion general that DM is an area around 2 types of data where at say the IMA or in exam assesment that would be a very basic awareness of the subject respectively.

M Cover

City/Greenwich Uni, Regional Key Skills Tutor SE & Cambs