In or Out of Batter's box

[rwest]

By rule, if a fair batted ball hits the batter while in the batter's box, it is a foul ball and a strike in FP if fewer than 2 strikes. What's considered in the "batter's box"? Do both feet have to be entirely in the box? Can any portion of one foot be outside the line and still be considered in the box?

Also, by rule the batter is out if they leave the box but then returns to the box and makes contact. Does the same definition for the previous question apply here? By leaving the box do we mean one foot completely out of the box? Both feet? Or any portion of one foot? When returning, does both feet have to be in the box?