One of the hardest calls for a basketball official is block or charge?

The rule book states:
A block is illegal contact which impedes the progress of an opponent with or without the ball.

A charge is illegal contact caused by pushing or moving into an opponent’s torso. The offensive player is required to stop or change direction to avoid contact with a defensive player that has legal guarding position (both feet on the ground and torso facing the opponent, and must keep verticality).

If a defensive player has legal guarding position and takes contact in the center of the chest from the offensive player, the majority of the time a charge will be called against the offensive player.

If a defensive player has not obtained legal guarding position and takes it off the shoulder, hip, or starts to fall before contact, the  majority of the time it will be called a block against the defensive player.