One way is using VLOOKUP. It looks through a vertical lookup list to select the corresponding cell.
A1 is the value to lookup (billing code)
$F$1:$I$3 is your lookup list. It can be on a separate sheet (I recommend a separate sheet for better management of your list). Note the $ (Dollar Sign) to make your lookup list not become relative as you copy your formula down and right.
2 is the column to pick your lookup from (in this example, 1 corresponds to col F, 2 to Col G, 3 to Col H, 4 to Col I)
0 is if your lookup list is sorted. If omitted, default is True (sorted ascending). If you haven’t sorted it, specify the zero. The lookup will find the first corresponding value in your lookup list. There are some finer details as to the way the lookup works. With a 0, you will get a N/A error if your item is not found in the lookup list. With a 1, the value returned will be the first value lower than your your value to be looked up. I would recommend a 0.
Edit/Addition: If the lookup table is on a separate sheet.
Sheet2 would of course be the name of the other sheet. I added a $ in front of the A1 ($A1) so that if the formula is copied across rows, the value to lookup will stay to Column A.