See, I think part of the problem comes from tags using different phrases for things that mean the same (eg, "part shade" or "part sun" = doesn't take a full day's worth of sunlight), but also because tags are usually put on plants that are shipped and grown in a wide variety of areas. For example, the Rhodies at our store are mostly labeled "Full Sun". Well, great if you live on the Oregon coast, cause they can use all the sun you got with the fog there. But here in our hot summer in the S. Oregon interior? Leaf burn, toasted plant, not the best place for a Rhodie. So I think the best way is a little research, which one way or another is going to come from someone else's experience. Wether you're referencing it in a book that describes local conditions (Sunset sometimes specifies how intense of sunlight certain plants can take, for example), looking up a plant online, or getting info from word-of-mouth, it's all coming from someone or another who tried it, or at least trustingly relies on someone else's information.

Like here, for example. Lots of people would be very happy to tell you in what kind of light conditions to put a certain plant depending on your area.

I'll always rely more heavily on a trusted fellow gardener's information than a little machine, sorry.

All my plants are already established for the most part. I was just basically curious in the device. I thought I would see what it said versus what I already guessed.
I am often on this site and have asked my share of questions of fellow gardeners :)
The only problems I have with that is, sometimes you ask about a particular plant and whether it should do good here and you get responses from gardeners in a totally different environment than you. Of course a plant might do good there, but not here or vice versa. I always try to keep in mind what zone they are from too.
Thanks for the info though.

You could look at it this way.... Full sun = Full sun more than 6-8 hours a day. Part Sun = more sun than shade, Part Shade = more shade than sun, shade = shade. That being said, most plants don't know what is said about them, and there is probably a fair bit of latitude to these definitions. If the plant doesn't seem happy, consider moving it to another location with more sun (or shade) whatever.

John 4b I love the way you broke that down! Have you personally used the SunCalc for your plants?

Here is a link that might be useful: SunCalc