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Думаю, я перепутал Invitation and Voucherом. But they are pretty much the same thing, it's proof you have a place to stay in Russia (and if the FSB wants, they have an adress to check up on you).

Yeah, Usually they are who invite or sponsor you. You can call or email or fax them and they should fax or email you back the proof that you will stay with them (usually its after you book your room...