From the way this appears in similar documents, it seems to be analogous to the way the general term "plastering" is used. It can also mean making rectangular borders and shapes with intonaco but I don't think there is any decorative meaning here. Like so many Italian rooms and spaces, the walls and ceilings are routinely covered entirely in plaster and this context seems to be a guideline for what a plasterer would do when making a standard estimate.
My attempt at a rough translation goes like this: Plastering of interior walls that are thicker than 15 cm will be calculated as if they are continuous surfaces, incorporating the cost for plastering the spaces that are not carried out, however, for surfaces greater than four square meters, the voids and missing areas are subtracted, and the plastering of those spaces is calculated separately.