I refer to this help page:

The description mentioned here doesn't really help fully.

If we go by this example - it won' t really help.

You can buy bulk from Amazon for trading purposes. But we pay it upfront - "then and there"; in fact, even before we receive the goods. Then we may bill it later to client when we do the sale of the Goods; in such cases, this example and description doesn't match. Therefore, this page needs to give still better understanding of the way Expenses and Bills work in detail! In fact, in so many ways, by hard experience I have learnt that BILLS and EXPENSES though sounding similar; cannot be used interchangably! Why that is so, this page should explain that in detail. Why you shouldn't use Expense in place of Bill and vice-versa!