"He writes my name on his book."
Translation:Ia menuliskan namaku di bukunya.
"Dia" is more common in terms of usage, you'll find "ia" being used mostly only on storybooks, novel etc. and certainly not in daily conversation, or you can say that it's more polite but the nuance and level of politeness are very subtle and in most cases are interchangeable. both can be used in formal circumstances