Hello,
the value 27648 is a digital value which representing the phyisical analog value. If you have a pressure transducer (4-20mA) and you will connect it to an AI. How the CPU sence the pressure. The changes in the pysical value (pressure) affect the electrical analog signal (4-20mA) and that happens inside the transducer. Then the changes of analog signal (4-20mA) is to be translated into digital values using the ADC "Analog to digital converter (0-27648). And that happens inside the Analog module. Then this value; the digital value is transferred to the CPU.
The reverse of the above senario is applied with Analog Outputs.

Siemens have also something important called the resolution of the analog channel ( min. 8 bits and max. 15bits)
The resolution is a factor when increase, then the precision increased. Thus because if you have 15-bit resolution AI module. for a very small changes in physical values, the CPU can mesure and sense that changes [i.e it would be able to measure (1/27648)*20mA.].
For 8-bit resolution; it would be able to measure (8/27648)*20mA.


I hope the above summary was useful for you to understand how the analog signals been read or written.

Also see the attachments and I am sure you will found in it more info.

Regards,