|First, let's get some terminology straight so that there is no confusion.|
In an Excel spreadsheet, they are called "cells", not "fields".
re: Function Codes
An Excel Function is an instruction to Excel to perform some built in task or calculation.
SUM, VLOOKUP, CHOOSE, MONTH, etc are functions.
A Formula is what a user enters into a cell. It will usually consist of 1 or more functions and will start with an equals sign.
"Code" is usually reserved those times when we're discussing macros, i.e. VBA Code.
Your example is typically called a formula, which in this case consists of 2 functions and a few arguments and it will be placed in a cell.
OK, now to answer your question...
Whether you need to enter this manually or via autofill will depend on where the formula needs to go.
Your example references D1. If you put that formula in a cell and the drag it down, the D1 will increment for each row, becoming D2, D3, etc.
If you drag it across to the right, the D1 will increment for each column, becoming E1, F1, etc. If you drag it to the right, D1 will become C1, B1, etc.
If you Copy/Paste the formula by copying the cell that the formula resides in (not the formula itself) it will change D1 by a row and column offset based on how far you are from the original cell. (Play with that to see what I mean).
So, we can't answer your question directly without knowing where these "preexisting entries" are. With more detail, maybe we could or maybe I've provided enough information here to help you accomplish your task.
Click Here Before Posting Data or VBA Code ---> How To Post Data or Code.