Abschnitt = part, not incision
darstellen = medical term which I don't fully understand myself, it seems to be used in the sense of "make s.th. visible or open s.th. up through incisions etc."
So something like: The alveolar process (bone) will be opened up/displayed by one or two mucoperiosteal incisions that only run over/above the alveolar ridge in traverse direction (i.e., don't extend further than that). The extension of the part selected for height increase determines if one or two cuts are necessary.
In case of alveolar collapse, or a alvolear atrophy that leads to a burr-like form (a sharp thin edge), [one cannot do the above procedure, so instead one must do this:] the alveolar process will be opened/up displayed by an incision into the alveolar ridge itself.