Some curiosity about multi-system training

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If I use multiple systems in the training , generate a well-trained model,then i will run dpmd. But How does my model determine which chemical environment my DPMD atom corresponds to in which system? As I see, DPData simply classifies atoms without making any distinction between similar atoms in mutilsystem. However, in the raw paper of DEEPMD-Kit, it encapsulates the chemical environment within the truncated radius of an atom well and corresponds to Ei.
So , i feel odd about that. Could any one tell me how dose this judge mechanism of multisystem work or where i can look up the code to find the answer?
thx.