What does this command do?

I have this code I'm trying to decipher but there are some commands I have no idea how to even start looking for the answer of what they do. I understand the first command is just a cumulative sum of the vector s_jt, but the next two I don't get what they're doing. cdindex is a vector of indices. I greatly appreciate any help.

temp = cumsum(s_jt);
sum1 = temp(cdindex,:);
sum1(2:size(sum1,1),:) = diff(sum1);

[Walter Roberson]

Are the values in cdindex consecutive?

[Sebastian Pinzon]

Not quite, it's an index of brand data (there's 24 brands) in different markets (94 markets). So cdindex looks like this: [24 48 72 ... 2256] and it's basically telling you the last row of the brand data for each market.

[Walter Roberson]

So the point of the code is to produce a subtotal of sales per brand?