"Natalia" would be the name she'd use in official documents like a birth certificate or passport. "Natasha" is a less formal form of "Natalia". It's indeed a d

"Natalia" would be the name she'd use in official documents like a birth certificate or passport. "Natasha" is a less formal form of "Natalia". It's indeed a diminutive but I wouldn't call it an affectionate diminutive like "Robert -> Billy". If I'm good friends with "Natalia" I'd probably call her "Natasha". If you drop the last "a", "Natash" sounds more affectionate. If you are a 5yo and a neighbour girl from a house across the street hit you with a toy shovel in the head you would probably come back running to your mom/dad, "Mom, Dad "Natashka" ("Natka") hit me with a shovel! She's baaaaaaadddd" :-)