The external interrupts are associated with specific interrupt pins. They can be set to signal on a LOW level, RISING edge, FALLING edge, or CHANGE (both rising and falling edge).
There is one "Pin Change" interrupt per port (8-bit group of I/O pins). The pins within the group can be individually masked. The pins are sampled periodically and the interrupt is signaled if any of the un-masked pins has changed. If you have more than one pin in a port enabled you have to read the interrupt flag register to find out which pin(s) changed.