Here’s what the GIRM says:
43…In the dioceses of the United States of America, they should kneel beginning after the singing or recitation of the Sanctus until after the Amen of the Eucharistic Prayer…The faithful kneel after the Agnus Dei unless the Diocesan Bishop determines otherwise.*
In other words, from the “Amen” until after the
Agnus Dei, everyone is to stand–there are no variations on this. After the
Agnus Dei, the default position is that the congregation kneels, “unless the Diocesan Bishop determines otherwise.” So if there’s no specific directive from the bishop, the posture is kneeling.
If the bishop has directed some other posture, then the congregation is to follow that directive (and here, the bishop may even direct that people stand, sit, or kneel at their own discretion).
- Here’s the part I excised: “except when prevented on occasion by reasons of health, lack of space, the large number of people present, or some other good reason. Those who do not kneel ought to make a profound bow when the priest genuflects after the consecration.”