This is called precedent Load, there's a good article about it here Preceding Load
Simply put, it's the action to use the previously loaded fields into another calculation without having to load a second table.
left(A,1) & right(B,2) as Key
Can be translated to
left(A,1) & right(B,2) as Key;
The bolded part is the preceding load statement and I only had to use one table to do what I wanted and it is as you stated, a type of resident load onto the same table from the previous loads.