In retail the word POS describes two separate things, and this confusion frequently turns into a problem in proposals. The first is the virtual POS: the infrastructure provided by a bank or payment institution that lets you take card payments from a website. The second is the store POS: the till on the counter, that is, the point where the product is scanned, the tab is opened, the receipt is issued and the shift is closed. This page describes the second. If your question is about taking payments from your site, virtual POS integration and payment reconciliation, that work runs under the payments and fintech heading. The only place the two systems intersect is bringing store sales and online sales together in the same report; clarifying in discovery which need belongs to which ends half the scope debate before it starts.
The real difficulty on the store side is not the till device itself but the gap around the till. The price on the shelf label does not match the price read at the till, because the increase was announced centrally but the label was never changed. The rule for a promotion sits in the cashier's memory, so the same discount is applied in two different ways in two branches. Branch stock reaches head office days later, and in the meantime a customer has been promised a product that another branch does not in fact have. Returns and exchanges are written in a notebook. When the till does not balance at the end of the day, finding where the difference came from usually turns into an archaeological dig that nobody undertakes. No single item in this picture is large on its own; together they mean the store's real performance is never seen clearly.
A Retail Management System (RMS) is this management layer around the store till. The till is the point of execution where the sale takes place; RMS is the whole that determines which price will apply at that point, which promotion will run, how stock will be deducted, which points the customer will earn and which figures will go to head office at the end of the day. Prices and promotions are defined centrally, can be differentiated by branch or region and are pushed down to the tills automatically. Stock movement is created at the moment of sale, and transfers between branches are recorded. Shift opening and closing and cash reconciliation are put on a standard flow, so a difference is visible in the shift where it arose rather than months later. Head office reports are also based on the record created at the till rather than on the branch's declaration; in multi-branch structures this is one of the most argued-about subjects.
Let us write down the limits up front as well. The device that issues the fiscal receipt is the new-generation payment recording device approved by the Revenue Administration (GİB); we are not writing software that replaces that device, we are writing software that talks to it. Which brand of device supports which integration method becomes clear in discovery, and this is the item that must be settled earliest in the project. Second, for a single-store business custom software is usually not the right decision; an off-the-shelf till programme gives a faster and cheaper result. Custom development becomes meaningful when the number of branches grows, when promotion rules fall outside the mould of standard programmes, or when the till has to share the same data with production, e-commerce and the dealer side.