Sheet stacking apparatus and an image forming apparatus

A sheet stacking apparatus, comprising:

• a first stacking portion on which a sheet is stacked;
• a second stacking portion on which a sheet is stacked, the second stacking portion being movable with respect to the first stacking portion;
• a third stacking portion on which a sheet is stacked, the third stacking portion being detachably attachable to the first stacking portion or the second stacking portion;
• and
• a restricting portion which restricts disengagement of the third stacking portion which has been attached,
• wherein the second stacking portion is movable such that the second stacking portion is located at a first position and a second position;
• and
• wherein the restricting portion does not restrict disengagement of the third stacking portion when the second stacking portion is located at the first position and the restricting portion restricts disengagement of the third stacking portion which has been attached when the second stacking portion is located at the second position.