Inside the 3Xx the GND and AGND are always connected via internal "star-point". This means both GNDs are connected on the circuit board at a specific location. The AGND is not filtered specifically special, but rather just like the GND there are multiple filter IC's to the housing. For this reason, the AGND must not be connected externally with GND.
Separating AGND and GND has the following reason:
Generally, all wires and circuits have a resistance. All load currents from the outputs flow through GND. Switching high currents may introduce strong interferences on the GND. Depending on usage the total current can be 0A ... 135A (theoretical current to the start point) (3XL with 9 output groups, 6x EB and 3x main board, all with max load of 15A).
Additionally, depending on the wire gauge and length there will be a voltage drop across the GND-wire. For example, for a GND impedance of 200mOhm Udrop = 0V (all outputs OFF) and Udrop = 2V for 10A output current.
A Voltage drop of 2V would be fatal for measurement accuracy of a sensor with Voltage output.
When only sensors are connected to AGND then the resulting current will be << and therefore the Voltage drops will also be <<. The Sensor signal is then independent on the load of the outputs of the controller.
For an actuator, such as a Valve, it is usually not important if there is a Voltage difference of a few Volts between the Valve GND and controller-GND, because it switches digitally or is current controlled.
For a sensor it is important that the reference GND at the Sensor is as close as possible to the reference GND of the controller. Additionally, every sensor supply (+Uext) has its own AGND assigned, to have the possibility to further isolate sensors producing interferences from those who are very sensitive.
CONCLUSION:
Sensors should be connected to AGND, independent of their power supply (Ubat or Uext), and may be further separated by using separate AGNDs for specific sensors.