written notice; letter of advice
A formal written notification, often used in business or official contexts to convey information or instructions.
We have sent a written notice regarding the change in the meeting schedule.
I received a letter of advice about the contract termination.
案内状 is an invitation or informational letter, often for events, whereas 通知状 is a formal notice, typically conveying decisions or changes.
Compound of 通知 (つうち, 'notification') and 状 (じょう, 'letter' or 'document').