Sheet stacking apparatus, sheet processing apparatus, and image forming system

A sheet stacking apparatus comprising:

• a discharge unit configured to discharge a sheet;
• a stacking portion on which sheets discharged by the discharge unit are stacked;
• a lifting unit configured to lift and lower the stacking portion;
• a first abutting portion configured to abut a trailing end of the sheets stacked on the stacking portion in a sheet discharge direction of the discharge unit, wherein the first abutting portion is configured not to move in an up-down direction;
• and
• a second abutting portion configured to abut the trailing end of the sheets stacked on the stacking portion,
• wherein the first abutting portion supports the second abutting portion such that the second abutting portion is movable in the up-down direction,
• wherein, in a case where the stacking portion is lowered from a first position to a second position below the first position, the second abutting portion is configured not to move in accordance with the stacking portion, and
• wherein, in a case where the stacking portion is further lowered beyond the second position, the second abutting portion is configured to move in accordance with the stacking portion.