By French law, service compris means a service charge (commonly around 15%) is already built into every menu price you see — it isn't added afterward as a separate line, and there's no obligation to pay anything beyond what's on the bill. Waitstaff are paid a proper wage rather than relying on tips to reach it, which is the same principle behind Switzerland's system, just built into the price rather than shown separately.
Because service is included, a further tip is genuinely optional. In Paris specifically — more than the rest of France — it has become common among both locals and tourists to leave a small pourboire (literally “for a drink”) for good service: a few euros, or rounding up the bill. There's no need to calculate a percentage.
Card machines in France generally have no separate tip field, so a pourboire is usually left in cash on the table or in the bill folder. Cafés and bars work the same way — rounding up to the next euro is standard. Tourist-heavy restaurants near major sights occasionally push harder for an additional tip than you'd see elsewhere in France; checking that service is marked as included on the bill before adding anything extra is a reasonable habit.
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