What is ARM CPU known state?

In the PSCI Specification, "CPU know state" is mentioned several times. For example,

• The caller must place all cores in a known state prior to the call. Like all other APIs presented here, the calls can only originate in the Normal world. On a SYSTEM_OFF call the implementation completely removes power from highest power level. The following start must be a cold boot from the point of view of the caller.
• One way in which cores can be placed into a known state is to use calls to CPU_OFF on all online cores except for the last one, which instead uses SYSTEM_OFF. If a UP Trusted OS is present, this method only works if the core that calls SYSTEM_OFF is the one where the Trusted OS is resident, as calls to CPU_OFF on this core return a DENIED error. Any core can call SYSTEM_OFF.

I can see one of the examples of the CPU known state is "CPU OFF". However, can not find a clear definition or requirement of the CPU's known state. Does anybody know what is the CPU's known state?