There are a lot of expenses that are allowable for tax purposes and a lot of expenses that are not allowable for tax purposes. Each country has its own tax rules and an expense that is deductible in country A might not be deductible in country B. For example, in the UK a quick test is whether the expense was wholly and necessarily incurred for the trade. For example, if you had to pay a fine because you submitted your tax return late, then this is not an deductible expense.
On the other hand, the following are allowable expenses: purchases, sundry expenses, interest on loans, electricity, rental, insurance policy and many many more.