It is very hard, or impossible, to deport someone to a country which does not have a deportation agreement with Sweden, or where there is no functioning state for Sweden to negotiate with. Such countries include Somalia, Afghanistan, Iran and Iraq, where a large proportion of Sweden's refugees come from.
"Roughly speaking around a third of those who have their applications refused are so-called hard cases, where someone refuses to travel and where there aren't documents to support carrying out a deportation," says the operative head of the Migration Agency, to Swedish Radio News.
Without identity documents a country may refuse to receive a person that Sweden claims is their citizen and their responsibility.
On Wednesday, Swedish Home Affairs Minister Anders Ygeman said the country could deport up to 80,000 asylum seekers and told police and the Swedish Migration Agency to prepare for such a task.
Ygeman based that number on the current rate of rejected applications, which is 45 percent.