What is the difference between Array1D & Array1S in the source code of EP?

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Dear all,

I am now learning the source code of EP.

I found there are two types of the array used in EP.

One is the "Array1D", and the other is "Array1S".

What is the difference between them?

I found it is mentioned that Array1S is "1D Slice Array" and Array1D is "1D Array". But I cannot understand the difference.

I searched on the Internet but couldn't have found any helpful information.

Any tips would be very helpful for me.