MERGE_2D merges:
+------------------<---------------+ | | ----->(MERGE_2D)--+--|elements|------------------>---+------> | | +--|single element|----|elements|--+
You can specify a box to select geometry to be merged. However, you cannot select vertices for merging, as they are automatically merged upon creation and modification of 2D elements.
MERGE_2D 100,100 250,357
Merges will only take place where they are feasible. For example, lines must follow the same direction and have a common point, and arcs or fillets must have the same center.