If they are pedigree, you'll need to identify them as such - I don't keep lops but someone on here will

hopefully they'll keep you right in that respect. BPA members have a code they need to include on the tag, so I'm assuming Lops are the same.
Otherwise it's normally UK, then your herd no and a number to identify the pig. If you're selling them and moving them to other holdings where they'll be raised for meat, I would just leave them without tags though - they can move farm to farm on a temporary mark (at least they can in Scotland - think it's still the same in England/Wales) and only having one tag/herd number on will make it easier when the time comes to slaughter.
I move weaners on a temporary mark and recommend the metal tags for slaughter (applied the day before so they don't get ripped out or grow into the ear

) or the button plastic tags for permanent ID tags.
HTH
Karen