S0:
From power on until the ICE starting for warming up (approx 12 seconds.)
You can enable the EV-Drive Mode during this S0, if the conditions are allowed so (refer to your owner's manual.) 
The EV-Drive Mode can not be turned on when:
	(1)the temperature of the hybrid system is high (ie; the hybrid battery is above 46C/114.8F),
	(2)the temperature of the hybrid system is low (ie; the hybrid battery is below 0C/32F),
	(3)the hybrid battery charge level is low (three bars or less),
	(4)the windshield defogger is in use or
	(5)the engine coolant temperature is below 0C/32F (maybe 20C/68F on the US Prius) 

S1a:
When the start coolant temperature is below 40C/104F, the warmup starts and continues for approximately 50 seconds after S0. 
You can not enable the EV-Drive Mode during S1a. 
The ignition timing is very late, after top dead center, therefore the engine does not have enough power to drive the Prius or charge the battery. 
Basically, the Prius can not use the engine power when you drive. Prius is driven by battery power and the battery is discharged. 
It is good idea to drive slowly or stand still warming up in this stage. 
When the start coolant temperature is between 41C/105.8F and 60C/140F, Prius is in this stage, but the engine does not start automatically. 
When you press the accelerator (including driving), the engine starts and runs for approximately 50 seconds or until the coolant temperature reaches 65C/149F. The ignition timing is very late too. 

S1b:
This is an extended warming up stage. 
You also can not enable the EV-Drive Mode during S1b on Gen3. 
The ignition timing is now normal, and the engine is able to provide power and charge the battery. 
You are now ready to drive without any penalty of battery discharging. 
	(A) If the the coolant temperature is still below 40C/104F after S1a, the warming up continues until 40C/104F. 
	(B) If the the coolant temperature is between 40C/104F and 50C/122F after S1a, the warming up continues until 50C/122F. 
	(C) If the the coolant temperature is between 50C/122F and 60C/140F after S1a, the warming up continues until 65C/149F. 

S2:
After the S1a or S1b, Prius goes to this stage. 
Prius acts as normal until the coolant temperature becomes 70C/158F (going to S3). 
The EV mode speed is limited below 10 mph on the NA Prius. 

S3:
When coolant temperature goes above 70C/158F, Prius goes to this stage. 
Prius acts as normal, but Prius requires an "idling check ceremony". 
The EV mode speed is now allowed up to 25 mph on the NA Prius. 
The "idling check ceremony" is done during an accelerator off gliding for several seconds on Gen3. 
The engine continues to run during the several seconds ceremony. 
After the ceremony, Prius goes to the S4. 

S4:
Prius now completed the warming up stages and in a full normal condition. 
if the coolant temperature is above 60C/140F on power up, Prius is already in this stage. 

NOTE: This is not a complete list yet and it is based on JP Gen3 Prius, therefore "subject to change" and may not apply other country's Prius.