What does "Parametric" mean in Photoshop Commands and Tools

Parametric means that adjustments are stored as text instructions, without actually altering the pixels themselves.

ACR works this way, but also adjustment layers in Photoshop. No pixels are changed until the adjustment layer is merged or the file flattened. Or the raw file rendered into an RGB file.

It's also known as "non-destructive", which is IMO not quite precise. A better term would be "re-editable", because that's what it essentially is, and that's the practical significance. As long as you keep the original state, no edit is really "destructive" as such.

EDIT a close second there...

Hi

The word is usually applied where the value to change an image is stored rather than the image itself being changed. This allows non destructive editing as the original image layer remains untouched.

As an example:

We could alter the brightness of an image with a curves adjustment from the menu. This would not be described as parametric.

A second method would be to add a curve adjustment layer. Here the image layer itself is unaltered but the instructions are held in the adjustment layer. This could be described as parametric.

The ultimate example of parametric editing would be camera-raw or Lightroom where the instructions to 'develop' a raw image are actually saved alongside the untouched image either as an xmp file, in a database or in a dng file.

Dave