The pitta sandwich is defined by the bread's pocket. A pitta is a flat round of leavened bread that puffs in a hot oven and, as it cools, leaves a hollow inside its own wall. Split along one edge, that hollow becomes a pouch the filling goes into, so the bread is not two faces stacked around a load but a single container holding it. That structural decision is the sandwich. The pocket is what lets a loose, mixed, often wet filling be carried and eaten one-handed without a base and a lid sliding apart.
The craft is the wall and the load it has to hold. The pitta is warmed before it is opened, because a cold pocket cracks and tears along the seam while a warm one flexes and stretches to take the filling. It is opened just enough to load and no further, since the intact wall is the only support the sandwich has and an over-split pitta is just a torn flatbread. The filling is packed in layers rather than tipped in, salad against one wall so it does not steam, the hot or sauced element kept off the seam so the soaked end does not give way. A pitta that goes wet at the base splits and empties, so the dressing is calibrated to bind without flooding and the heaviest, oiliest part of the filling sits deep where the bread is strongest.
The variations are a takeaway map of what the pocket can hold. Falafel with salad and a tahini or chilli sauce is the meat-free standard; doner or shish kebab meat with salad and garlic sauce is the late-night build; halloumi, hummus, or a plain salad pitta keep the same logic with a lighter load. The wider flexible-bread family, the wrap and the flatbread roll, solves the same containment problem with a fold instead of a pocket. Those deserve their own article rather than being crowded in here.