The evocati augusti were soldiers of the Praetorian Guard who had obtained an honorable discharge after serving for their time and chose to re-enlist.
Due to being evocati, soldiers who had re-enlisted after serving their 16 years, of the Praetorian Guard, they were the only evocati allowed to use the adjective of "imperial" (augusti).
[1] Unlike the regular evocati, it appears that evocati augusti were hand-picked by the Emperor, rather than it being open for all.
[2] Many evocati augusti went on to be centurions in the regular legions, and beyond.
This article about the military history of ancient Rome is a stub.