The STM8S external interrupt configuration is very limited (one interrupt vector per port A through E). As long as just one GPIO triggers the interrupt one knows implicitly what caused it. When more than one input is used it's not so easy.
The STM8L applies this scheme only to the ports B and D. A second set of interrupt vectors capture interrupts triggered by port inputs Px0 through Px7 for ports A through E (plus port F in the case of PF0). It's thus possible to get two interrupts for the same port B or port D change, or (which is more useful) to have two different interrupts for, say, PB2 and PC2.
The STM8L family has more options than in the STM8S family but still one has to be very careful when designing a circuit.