Thanks for explanation.
Since asking the question I've been implementing External Data tab.
As mentioned previously, I'm using temperature dependent heat generation. Once I created the 'External Data' module and uploaded an external .csv file (which has 2 columns - Temperature and Heat Generation), I linked the 'External Data' module to the Setup tab of the Transient Thermal Module. After updating the External Data module I opened Setup tab in the module. I then inserted Heat Generation in the Imported Load section (as shown below)

I selected the 2 bodies in my simulation but am not able to 'import load'.

With respect to using APDL Commands: I'm not sure how to implement the code. I understand I need to create the table but do I create the table in the script or upload an external .csv file. So far I'm using the method as shown in the image below.
I'm still confused about the command on line 12. Plus, how are the units for TEMP and HGEN defined? For me they are [C] and [W/m3].
What exactly am I doing wrong and how to rectify it (for both methods if possible)?

Thank you so much!