I'd like to know if we can add a custom field to customer to store all the product line (class) for purchased items please? I know there is a subtab named Items/Purchased but I want multiselect custom field for that Any help would be appreciated

Let me understand this: On a Customer you want a field called something like "Purchased Product Classes". In this field the user or a backend process will multi-select the Classes of all the items the customer has previously purchased. That can be done or course. Can I suggest a slightly different idea? How about using a new Sublist based on a search of the purchased classes. It could sit next to that native sublist you mentioned. The other advantage of this approach is that you can then also include $ amounts and Quantities listed by Class. Something like this.

Thank you @Netsuite Tragic It looks great. However custom field is needed to integrate

Are you able to create the multi select field in the first place?

yes @Netsuite Tragic

ok, so you start with a saved search to return to you a Customer IDs and Classes. then your script will go through the search and set the field you created

Thank you @Netsuite Tragic