Differentiate the compound words

In accounting, we use compound words like "electricity expense", "utility expense", "salary expense", etc. These all belong to one category of operating expenses. I also have compound words like "deferred expenses" and "prepaid expenses" which belong to the balance sheet.

Is there a fundamental difference between phrases like "salary expense" and "prepaid expense"? Is the below statement correct?

"Salary expense" is a compound noun where "salary" is a noun and "expense" is a noun, and "prepaid expense" is a compound noun where "prepaid" is adjective and "expense" is a noun.