When I do xml=T on customer record, I get "addressbook" sublist in that I get <addr1_initialvalue>test address 1</addr1_initialvalue>, I am not sure what is "addr1_initialvalue field is? is anyone aware of this field? Also I can use this field in my code?

Why do you want to use this field value instead of "addr1"?

@NElliott Right, Because value for "addr1" is not getting in SuiteCommerce Advanved, I am calling Suitelet from SCA webstore, I'm aware that this is an old issue with SCA, They did something recently, I'm not sure when exactly I believe it was in 2022.1 release, and now I'm getting the correct value for "addr1_initialvalue". I just want to know is it safe to use "addr1_initialvalue" for Address Line 1.

No I'm not aware of that field, if it's documented and NS are saying you can use it then I imagine it's OK.