you can call your boyfriend your sweetheart, but not your other friends. "Sweetie" "Sweetheart" are also somewhat generic terms of endearment and are heard between others but the use is culturally dependent. As a foreigner you should use the term sweetheart for your boyfriend and not waste your time figuring out the other stuff. Sweet- used to describe food or people with a friendly helpful disposition

I think the words are used in the culture and custom conditions, Not have the absolute right or wrong. When you say the words somebody can understand your means. P S: If you want to do the academic, you must be attention use the words condition.