Any particular industry? I don't have any experience in this myself, but have seen enough of them used to know that POSs can differ greatly among industries. The basics include knowing about product SKUs, credit card and cash transactions, credits, refunds, discounts... stuff like that.
Industry-specific POSs have their own requirements. Restaurants, for example, will need to support touch screens, and possibly incorporate table layouts and easy selection by servers who are moving fast, fast, fast. They also need to be able to control a cash drawer to unlock it at the end of the transaction. Some POS systems have card security, too, for each transaction (again, restaurants). And since I'm on talking about restaurants specifically, but I suppose it could apply to other industries, once the transaction is entered, something may need to happen with it. In a restaurant, the order needs to get sent to the kitchen. That's before the transaction is complete. In other systems, the order probably needs to get sent to the warehouse. Some POSs incorporate inventory management so each sale reduces inventory. Then there's reporting (end of shift, nightly batch, credit card vs. cash sales, etc.). And don't forget connectivity to accounting systems (QuickBooks, Microsoft Dynamics, etc.)
This is off the top of my head, so I'm sure I am only scratching the surface.