Liturgically, EF, deacon and subdeacon; liturgically, OF, deacon or concelebrant, incense, clergy chant their parts; musically, in the Austrian tradition, a larger orchestra and longer movements in contrast to the Missa brevis
A solemn Requiem Mass would include neither Gloria nor Credo. The inclusion or not, of those prayers would depend on the Mass formula. Thinking of Lent, for example.
Madorganist put it succinctly for TLM ...Liturgically, EF, deacon and subdeacon, incense, clergy chant their parts. I might add Gospel procession and other ceremonies.
I've often wondered what officially makes a solemn Mass for the modern form. Hadn't realized a concelebrant might be part of it.
Ash Wednesday could be a Solemn Mass, and clearly it wouldn't include a Gloria.
As to the modern form, I believe the distinctions between Sung and Spoken Mass exists, but I further believe that the distinction between High and Solemn High was formally abolished (but I can't find the footnote which proves it just this morning).
I should have clarified I was talking about non-chant musical settings, which I perhaps mistakenly took to be the implied context of the original question. Usually composed Requiems are titled as Requiems and the traditional lack of instruments during Lent means accompanied settings of a Missa Solemnis would not be used except for permitted exceptions.
And yes the walled distinctions between musical degrees of Mass does not exist in the reformed liturgy.
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