The major difference between revenue and gains (or expense and losses) is that revenue and expenses are in direct result of an entities' main business operations or central business purpose. However, gains and losses are from incidental transactions of a business entity.
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For instance, revenue for a business may be realized through selling on profit while gains may be in the form of an increase in the value of an asset. On the other hand, the expense may be salaries paid to workers while the loss may be in the form of losing a lawsuit and paying compensation.