Normally you're not allowed to have multiple static translations with the same local address.
One of the uses of "extendable" is to allow two inside global addresses for a single inside local address
Like,
ip nat inside source static x.x.x.x y.y.y.y extendable
ip nat inside source static x.x.x.x z.z.z.z extendable.
When a packet is coming from outside to insde with destination address y.y.y.y or z.z.z.z, it will be sent to x.x.x.x
When a pkt is going out, it will have src address as y.y.y.y
The reason why you don't see "extendable" when using interface option is because you are just using one IP then.
