In OctoPrint when receiving temperature data (M105) what is T0? I've setup OctoPrint with the goal to log temperature data of my hot end. So, I've enable serial logging and examined the serial log file.
The Output for an example line is as follows:

Send: M105
Recv: ok T:20.7 /50.0 B:20.0 /0.0 T0:20.7 /0.0 @:0 B@:0

So, from what I understand:
T = Thermistor temp of hot end °C
In the example, current temp is 20.7 and target temp is 50.0
B = temp of bed °C
In the example, current temp is 20.0 and target temp is 0.0
But what is T0?
Or the @ and B@?
T is the selected tool, T0 is the first hotend tool. If you only have one hotend, T and T0 are exactly the same. 

Do note that G-codes are described on the G-code wiki page, for M105 you can find:


  The parameters mean the following:
   - T, T0, ..., Tn - extruder temperature. In a single extruder setup, only T will be reported. Some firmware variants will report no T0 in multi extruder setups - in that case T is to be considered the temperature of the first tool. Otherwise, T should be considered the temperature of the currently selected tool (which will be repeated in one of the Tn entries)
   - B - bed temperature
   - C - chamber temperature
   - @ - Hotend power (Prusa only)
   - B@ - Bed power (Prusa only)
   - P - PINDAv2 actual (Prusa MK2.5/s MK3/s only)
   - A - Ambient actual (Prusa MK3/s only)