From Ceremonial of Bishops:
“166 When the bishop returns to the chair after the communion, he puts on the skullcap and, if need be, washes his hands. All are seated and a period of prayerful silence may follow, or a song of praise or a psalm may be sung.”
(Ceremonial of Bishops, Liturgical Press, 1989, ISBN 0-8146-1818-9, page 60.)
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from romanrite.com/girm.html :
“112. … At a Mass celebrated by the Bishop or at which he presides without celebrating the Eucharist, the norms found in the Caeremoniale Episcoporum should be observed.”
The “Caeremoniale Episcoporum” is the Latin name for this liturgical book, Ceremonial of Bishops.
From the GIRM:
“42. … A common posture, to be observed by all participants, is a sign of the unity of the members of the Christian community gathered for the Sacred Liturgy: it both expresses and fosters the intention and spiritual attitude of the participants.
43. The faithful should stand from the beginning of the Entrance chant, or while the priest approaches the altar, until the end of the Collect; for the Alleluia chant before the Gospel; while the Gospel itself is proclaimed; during the Profession of Faith and the Prayer of the Faithful; from the invitation, Orate, fraters (Pray, brethren), before the prayer over the offerings until the end of Mass, except at the places indicated below.
They should, however, sit while the readings before the Gospel and the responsorial Psalm are proclaimed and for the homily and while the Preparation of the Gifts at the Offertory is taking place; and, as circumstances allow, they may sit or kneel while the period of sacred silence after Communion is observed.”