Income Statement - Chart of Accounts

CFA Level II FRA
#1

Where can I find official guidance on how to construct a COA, specifically the part about OpEX?

I have seen chart of accounts where the income statement is

  1. OpEX
    A) Sales General Administrative
    i. sales
    ii. general and administrative
    B) R&D

I have also seen

  1. OpEX
    A) Sales
    B) General Administrative
    C) R&D

Are there hard and fast rules on this and if so, what’s the primary source on this?