The SEPO (Security EPOch) tag specifies the minimum Security Epoch required for the device to run the provided image. This allows Apple to prevent devices from running previously vulnerable loaders. An example of this is when Apple increased the SEPO on iPod touch (2nd generation) MC models in addition to patching 0x24000 Segment Overflow to prevent the use of the ARM7_Go vulnerability from the 2.1.1 loaders.