Why is Depreciation & Amortization embedded into COGS/SG&A/D&A?

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Depreciation & Amortization can be treated in nearly infinite ways depending on the business. 

Here's a simple example that helps me: if a company buys a cash register that can be used for many years, its depreciation expense will be recognized as an SG&A expense because the register is used in the selling of goods. 

Wait until this guy finds out about goodwill amortization

Drug: Accounting

Street Name: “fun little tricks & tips on tax avoidance”