People were issued a Sperre for many reasons: indispensable to the war industry, membership of the Jewish Council, holding a passport from a 'friendly' state etc. In October 1942, 17,000 Jews thought they had the right papers.
The exemptions issued, however, were all marked 'bis auf weiteres' [until further notice], which meant that they were only temporary. In fact, the exemptions served to spread the deportations and to keep those remaining behind under control. In the end, virtually everybody was deported. Only a few protected groups made it through the war, and that was because of various coincidences.
Article
Sperre
Document granting temporary exemption from deportation.