Hi,
The auto-discovery is to discover the remote PE.
With Martini, the remote site has to be specified using neighbor x.x.x.x. This is not required in BGP L2VPN / VPLS.
For the sites to be unique, a site identifier needs to be configured.
You can have same site-identifier on 2 PEs if you need VPLS multi-homing for instance.
Cheers,
Ashvin