‘*Here in the picture*:, in the Sublist i have sourced the ‘_item_’ list as a sublist field. I wanted the other fields (to be sourced)like ‘_rate_’, ‘_amount_’ , ‘_itemType_’ to be sourced according to the item i chose in the ‘_item_’ field (from the respective item record). Please provide a Solution , thank you in advance.’ 🙏

I already replied on the site.. but since you posted here its easier to have convo...

You could apply a client script to your suitelet, and use the entry point fieldChanged to populate the other values.  Basically when the item "field" is changed, source the other values and populate.