In Attack Position (called Attack Mode in the anime), a Monster Card is face-up, in a Vertical Position. In this position, the ATK value is used to represent this card, except where a card effect requires a specific examination of the DEF value.
Attack Position monsters should always be face-up. The only time when a face-down Attack Position Monster Card is acceptable is when it is placed that way by the effect of the card "Darkness Approaches".
Gameplay and Glossary