help using tap device

Hi,
I'm trying to pass ethernet frames from a user process over the wire,
using the linux tap device. I see a lot of docs/threads about using tun
device, but none for tap device.
If there are multiple physical interfaces, om which phy dev is a
message sent out, that is eritten using the tun device?
Does anything need to be configured (like iptables??) or just open a a
tap device, and write to this?

thanks