Utilities / running costs (Nebenkosten).
Running costs on top of net rent: heating, hot water, building maintenance, common electricity, often internet/TV.
Nebenkosten (NK, "side costs") are running costs on top of the net rent. They typically include heating, hot water, building caretaker services, common electricity (stairwells, lifts), elevator maintenance, water and sewage. Internet/TV/radio are optional per contract.
Gross vs. net rent: Swiss listings often show "CHF 2,400 / month". That's usually the gross rent including the NK advance payment. Important: clarify how much is net rent and how much is NK advance. The 3x rule typically refers to the gross rent.
Reconciliation: NK are usually paid monthly as advance payments and reconciled once a year. Additional payment or credit possible. With NK advance of CHF 250/month = CHF 3,000/year; reconciliation can result in ±CHF 200-500 difference.
Lump sum vs. advance: with lump-sum NK you pay a fixed amount without annual reconciliation. With advance NK the actual usage is reconciled. That is usually more tenant-friendly, because lower usage gets you a credit.