expression
currently away from one's seat; AFK
Used in online chat, messaging, or office contexts to indicate temporary absence from one's desk or keyboard. Often seen as a status message.
すみません、離席中です。後ほど折り返します。
Sorry, I'm currently away from my seat. I'll get back to you later.
チャットに「離席中」と表示されている。
The chat shows 'AFK'.
不在 means absence in general; 離席中 specifically refers to being away from one's seat or desk.
Compound of 離席 (leave one's seat) + 中 (in the middle of).