All External / Environment Activities should have the same size in height and width. If a bigger size is needed, the respective size should be a multiple of a pre-defined basic size.
Data sources (Inputs) shall be placed on the left side, data sinks (Outputs) shall be placed on the right side of the Activity Chart - even if their names are identical.
In addition to the External / Environment Activity name it might be useful to add textual notes next to the name to indicate their origin from the modeling point of view - e.g. TESTBENCH.