Registered mail — a postal service where items are recorded/registered for secure delivery (registered post).
重要な書類は書留で送った方が安心だ。
It's safer to send important documents by registered mail.
郵便局で書留を出すと控えの番号をもらえる。
If you send registered mail at the post office, you receive a tracking number receipt.
海外へ書留で発送した荷物は追跡が可能だ。
Parcels sent registered overseas can be tracked.
The act of putting something on record or writing it down — noting or recording information.
大事な点はノートに書留しておいた。
I wrote down the important points in my notebook.
会議の内容はすべて書留しておく必要がある。
The contents of the meeting need to be recorded/written down completely.