Additional means coming on top of what is already there. An additional charge, additional information, additional staff.
It is the formal word where extra is the plain one, and it belongs to bills, forms and contracts for that reason. An additional charge always means you will pay more, and the word is chosen because it sounds less like it.
1adjectiveadditionalComing on top of what is already there.
“We need two additional chairs for the guests who are coming late.”
additional is a quality.
Latin
additionalis
English, today
additional
Learned borrowing from Latin additionalis.
Etymology from Wiktionary, CC BY-SA