After you save an image as a regular JPEG file with a specific level of compression?

Your other cross-post also has relevant information too:

https://community.adobe.com/t5/photoshop/with-jpeg-images-saved-from-photoshop-how-can-you-tell-low-compression-from-high/m-p/10802614#M290696

If you resave a Photoshop JPEG, it does indeed know the previous compression level. This info is saved as metadata in the file. ExifTool reports the following for a baseline level 10 compressed JPEG:

[Photoshop] PhotoshopQuality : 10
[Photoshop] PhotoshopFormat : Standard
[Photoshop] ProgressiveScans : 3 Scans

There is no single value stored in the jpeg file. Instead tables are used for the quantisation and encoding used. These are used during the decode. So Photoshop's nominal 1-12 settings result in different sets of values used in the quantisation and encoding and these are stored in the tables.

See here for detail:

https://en.wikipedia.org/wiki/JPEG

Dave

If I recall correctly, former Photoshop senior engineer Chris Cox once said here that a jpeg should automatically resave at the same compression level it was previously saved at. If that's correct, it must be somewhere in metadata.

Of course, you should never resave a jpeg.

I think that information about level of compresion is not stored in JPEG file.