Lightroom 'PREVIEWS' are the means by which non-destructive editing is made possible. The LR catalog stores all imported files and their changes within it and the previews there by leaving the original file unaltered.
Take a quick look above at the catalog and preview files created when a LR catalog is created. It is within these files that the meta-data of the original source files (lrcat) and its previews (lrdata) are created.
Lightroom provides 4 modes in which files can be imported into previews. I have copied below what Adobe says on their site. I have marked them with what it means to a user's workflow.
Bit more about them
Snippet below from Adobe:
WHAT TO USE?
Well as you can see from the above documentation there is a clear route to go for faster import time and later taken on additional rendering when working on an image (or) go for a slower import and have all files in their full glory to edit. So is this binary decision? No! One can always blend both of these options and I share below the route that I have now adopted.
Before my approach is shared you need to know what my requirements are. My requirements (that is a result of some hardware limitations):
Quicker import time is always good - however to be very honest it is not a deal breaker. Faster is better.
Catalog size is very important - I work on a Macbook Pro with a 128 GB SSD + 8 GB RAM. Now you see how dear, space is to me!
Ability to review and cull images needs to be as quick as possible. This is of paramount importance. With a lots of images from wildlife shoots, the ability to quickly sift through images (my first pass is always to check quickly without even zooming in) is very critical. That way I get rid of junk as quickly as possible.
Now that you have a better understanding of what my requirements are, you will probably appreciate the approach below.
Import images always with 'Embedded&SideCar' preview - reason being these are quick imports with reasonable size (better than minimal) which is dumped by the camera itself.
Unlike Minimal this does not automatically generate larger standard previews. These get generated when I explicitly enlarge them and work with them on the Develop module. LR will only create the larger size cached images for these that I intend to work on.
Being small images I can quickly go over them via the thumbnail strip at the bottom (in the Library module) and using the previews rate/flag images for culling later.
Once I need to work on the right images, I enlarge them in the Develop module and pretty ok to have a few seconds for it to load the size good enough to start my edits.
As the previews are smaller and I generate large size images only during editing my catalog size can stay very controlled.
As long as I do not edit images in Develop, LR will still fall back to the smaller sized thumbnails built using the Embedded & Sidecar previews.
If this is still not clear, do drop me a msg on my instagram @vijayramh. Thanks!