The division that organises Sufi practice is whether the remembrance is voiced.
Silent dhikr is associated above all with the Naqshbandi order, which holds it to be the higher form on the ground that it cannot be displayed and therefore cannot decay into performance. The practitioner places the formula in the heart, coordinates it with the breath, and in some lineages moves it through named points of the body. The practice is undetectable from outside and can be sustained continuously in ordinary company, which is precisely the intended advantage.
Voiced dhikr is collective. A circle sits or stands, a leader sets the formula and the pace, and the phrases shorten as the session proceeds, often reducing to a single syllable or to the sound of the breath itself while the tempo rises. Bodies move with it: swaying, rocking, a turn of the head from side to side. Sessions of an hour or more are usual, and some orders add drums.
What the two forms share is that the text is assigned. A teacher gives a particular disciple a particular formula, a particular number and a particular time, and the assignment is personal and adjusted as the disciple progresses. That is the feature most sharply distinguishing Sufi dhikr from the general Muslim practice of remembrance, which any believer performs without authorisation from anybody, and it is why the orders are built around a chain of transmission rather than around a congregation and a building.