One the Mac: Holding down option key & pressing - or + key, what does this do?

The title says it all.
While cruising thru some RAW files I meant to press the Commend key & minus or plus keys to reduce or enlarge the image but accidentally held down the option key instead.
I noticed that some image became lighter and some darker depending wether the minus or pluse key was pressed.
Did a couple of searches but could not find a direct reference to this action. What is happening here? What is this controlling?
Is this just a viewing option or this changing the file?
Are these changes displayed somehwere within the interface?
Thanks

Raw files are _never_ changed in FastRawViewer, FastRawViewer opens raw files in read-only mode.

The Option key (or the Alt key, if using Windows) pressed together with the + key is the same as Menu – Adjust – Adjust exposure correction – Increase Exposure; pressed together with the - key is the same as Menu – Adjust – Adjust exposure correction – Decrease Exposure; please see the manual that comes with FastRawViewer, the section titled 'Manual Exposure Correction' (page 124), it also mentions how the change is reflected in the user interface (lower window bar). The manual is a regular fully searchable PDF, and can be accessed via FastRawViewer main menu -> 'Help' -> 'PDF Manual; or you can download the manual separately from FastRawViewer download page,
https://www.fastrawviewer.com/download
Direct link to the manual is:
https://updates.fastrawviewer.com/data/FastRawViewer2-Manual-ENG.pdf

"Raw files are _never_ changed in FastRawViewer, FastRawViewer opens raw files in read-only mode."

I assumed that this would be your practice, thank you for the confirmation. I noticed after posting that [file name].xmp files were appearing in the foldr with the images, I assume that this is how you handle nondestructive editing.
My searches were ineffective as I could not quite figure out how to phrase the search plus the Mac uses different names for keys, such as Option instead of Alt. I often forget to include that in searches.
Thank you for the direct link to the manual.

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