Grouping rows based on column values and comparing row values

Hi there,

Please see attached. Single Header row in row1. Data begins in A2. I originally thought I would need to name the rows to enable an easy way of writing the calculation formula but with unknown numbers of rows to name or group, I don't know how that would have worked.

What I am looking to do is calculate (columnC row? minus columnC row?)between:

(i) rows that have a column E value of "NB";
(ii) rows that have a column E value of "S"; and
(iii) only perform this calculation between rows that share the same value for columns A,B & D ; and
(iv) where the column C value for Step(i) rows > column C value for ANY Step(ii) rows, populate Step(i) rows column F with "S"; and
(v) where the column C value for Step(i) rows <= column C value for ALL Step(ii) rows, populate Step(i) rows column F with "NB"

So, the final result in column F should be "S" in rows 5&12 and "NB" in row 17.

Thanks

Trent