For the Four Mandala Offering of Zabtik Drolma, which is the most popular such ritual amongst Kagyupas and Nyingmapas I think, one offers a Kartor (or "White Torma") during the torma offering. The Shelzay torma is a separate torma offering of "Food" as one of the 8 outer offerings.
It's customary to offer a new Kartor each day, or each session, and to take the offered Kartor out before one starts a new session. This is the general rule, for any daily deity practice where one offers a torma of some sort.
HOWEVER, in the context of drupchos or drupchens, there are occasions where the torma offerings are left on the shrine, and new ones are added with each session. These can be special sorts of tormas, that end up being a big pile by the end.....
Additionally, some "Nyendrup" practices--intensive accumulation retreats--require that one have a main "Phuktor" or "Tentor" and that one add a small "tebkyu" or dough triangle" to the plate of the Phuktor or Tentor for every session.
There are many different methods and ways of offering, we can say.
In daily practice, I offer biscuits or crackers, and I take them out after the session, or prior to the next session. This was specifically explained to me by my gurus as being appropriate, if one didn't have time for making Kartors, Martors, etc.
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