Following is the code used in one of the program under for loop. What does it mean? What does square bracket mean?

P{a1}{a2}{a3}{initialStateIndex}(:)=0;
P{a1}{a2}{a3}{initialStateIndex}=[];

[Stephen23]

Square brackets explained in the MATLAB documentation:

https://www.mathworks.com/help/matlab/matlab_prog/matlab-operators-and-special-characters.html