Tokyo Dome Seating

I just bought tickets for the World Baseball Classic- SK vs Australia on Monday. Can anyone explain the seating though? I got the tickets from StubHub but the seating assignment just says “Infield A”. There’s no row or seat numbers? Do you sit wherever? Are you assigned seats upon arrival?

Confused bc other games there were tickets selling that had a Row # and seat # so I’m not understanding why this game just says Infield A. TYIA