Why wICA method use multi channel EEG signal over single channel EEG signal ? Which channel is subtracted from {data1 = [d1;d2;d3;d4;d5;d6];} and why??

eeg1 = data{1,1}.X;

eeg2 = eeg1.';

d1 = eeg2(1,1:10000);

d2 = eeg2(2,1:10000);

d3 = eeg2(3,1:10000);

d4 = eeg2(4,1:10000);

d5 = eeg2(5,1:10000);

d6 = eeg2(6,1:10000);

data1 = [d1;d2;d3;d4;d5;d6];

%% Apply WICA

[wIC, A]= wICA(data1,[],8,0);%[wIC,A] = wICA(data,[],5,1);

artifact = A*wIC;

clean = data1 - artifact; %In here ,which channel is subtracted from {data1 = [d1;d2;d3;d4;d5;d6];} and why??