What is a server-client POS?
A server-client POS has one till that holds the shared database and other tills on the same local network that work from it, so tables and orders stay in sync.
When a business has more than one till, the tills have to agree on what is happening: which tables are open, which orders are held, what has gone to the kitchen. In a server-client set-up one machine is the server. It holds the database. The other tills, the clients, connect to it over the local network and read and write the same data.
The main advantage is speed and independence from the internet. Clients talk to the server across the shop's own network, often in milliseconds, so a server can open a table on the bar till and the floor till sees it straight away. If the internet drops, the tills can keep sharing orders locally; the server syncs with the cloud when the connection returns.
The trade-off is that the server till matters. It should be a reliable PC on a wired connection with a fixed (reserved) IP address, it should not be turned off during service, and it should be on a UPS if the power is unreliable. Clients need to be on the same network, and the router must not isolate devices from each other.
SeroPOS has Server / Client Mode in the app on the Pro plan, under Admin, then Tools. Database Mode can be Standalone, Server (Windows) or Client (any platform). The server shows its address and access token and opens a 24-hour setup window for new tills, which 'Lock Down Now' closes. Clients find the server on the network, or you enter its address, port 19091 and token, then Test Connection. Floor plan, holds and KDS are shared.
A pub runs three tills: a Windows PC behind the bar as the server, and an iPad and an Android tablet on the floor as clients. A server opens Table 7 on the iPad, the bar till sees it as Occupied at once, and when the internet fails at 8 pm all three tills keep taking orders and printing kitchen tickets. Sales upload to the cloud when it comes back.