From a Systemic Functional Grammar (SFG) approach, the present descriptive study (Matthiessen, 2006, 2007; Caffarel, Martin & Matthiessen, 2004) is aimed at characterize the specialized oral and written discourse, from the probabilistic variation that present of the realizations of the obligation system in Spanish, which tend to become critical features when characterizing oral and rather specialized written language. To achieve our objective, we take as context of observation a multiregister corpus, characterized by the crossing of the oral/writing and degree of specialization variables, that is, from the register construct perspective (Halliday, 1982a), mode and field respectively. The automatic exploitation will be carried out through the linguistic tagger and query interface, El Grial (Parodi, 2007), which allows us to access the absolute frequency of instantiation of each term in the systemic network of obligation. Upon this frequency, we estimate the conditioned probability of occurrence and, upon the latter, the obligation system variation in the corpus context. For this last calculus, we assume an error estimation of 5%. Results show that there is a direct proportional correlation between the degree of specialization and the degree of obligation present in the different registers of the corpus.