The origin of Fluentd’s .pos file name seems to be “position”.

What is a .pos File?

I’ll quote from the official Fluentd documentation and a Qiita article that explains it clearly.

This parameter is highly recommended. Fluentd will record the position it last read into this file.

• Source: tail Input Plugin | Fluentd

Agent-type log collection tools periodically record how far they have read the target files for monitoring. That file is called a pos file. By checking the contents of this pos file, you can confirm how far writing to storage has been performed. (「エージェント型ログ収集ツールは、定期的に監視対象のファイルをどこまで読み込んだか記録します。そのファイルを、posファイルと呼びます。このposファイルの中身をチェックすれば、どこまでストレージに書き込みが行われたか確認することができます。」)

• Source: ファイルがどの位置までログが転送されたか確認する方法（fluentd/FlyData） - Qiita

Other Meanings of POS

My American engineer colleague taught me some POS slang.

Since it’s not a very good meaning, when talking about Fluentd, it would be good to clearly state “pos file” so you don’t make the other person feel uncomfortable.

piece of shit 《a ～》〈話・卑俗〉くそ（野郎）、ごみ、役立たず、粗悪品

• piece of shitの意味・用例｜英辞郎 on the WEB：アルク

Abbreviation for "Parent Over Shoulder".

Literally translated as “parent over shoulder”, meaning “parent is behind”, “parent is in the same room”, “parent is right nearby”, etc.

• POS の意味・用法・例文|英語ネットスラング辞典

That’s all from the Gemba where I wanted to know the origin of pos files.