depends on the plant.

depends on the sun.

depends on the amount of rain. (No matter how much you water, even though it keeps plants alive, it just isn't the same.)

may depend on the phases of the moon or the tides or El Nino.

The only "full sun" recommendations I take at face value are for the yuccas.

Some of my irises, coneflowers, etc. grow & bloom right beside the hostas, pigeonberry, & aucuba that need bright or filtered shade.

This year I'm trying tomato plants again, even though I swore off them last year, when I got not one tomato off any of the gorgeously lush green plants that I watered assiduously even when I was wilting myself.

but I'm trying them with a difference-

some full sun, some almost full sun, some morning sun.

So far, the most robust ones are thriving where they get morning sun & maybe an hour of afternoon sun & then shade.

Their closest companions are the (shade-loving) vinca minor.

Go figure.

When I first started gardening, all those terms were a little unclear also. However, based on experience and lots of reading, this is what I have learned:

a) full sun means that about 6-8 hours of sun is sufficient. Again, for some plants like veggies, more sun is better, but it doesn't have to be all day sun to thrive and produce well. For example, in your Texas growing conditions (OP), you are probably getting about 12-14 hours of sun if your yard were fully open to sun all day long.

b) partial sun - means morning sun and afternoon shade, not filtered light in a mostly shady spot like I used to think

c) plants that like shade -- some plants are supposed to do fine in shade or even want only shade, but other plants that can handle shade, if you give them just a bit of sun, either in morning or afternoon, and they do even better. an example is impatiens and purple queen. impatiens also did better with filtered sun than with just plain old shade like the label says that it can only do shade

d) fuchsia - no matter what the label says, it can't grow in my area even in shade, so forget it every year when they show up in the nurseries

In my view, for edibles at least, "full sun" in Texas is sometimes asking for trouble. My melons love it, but my tomatoes would rather not be in full sun for the hottest parts of a very hot day. In fact, although my melons get full sun, all my other veggies get 5-6 hours of sun and do great.

Note that "shade" can mean different things. It can mean being covered to the extent the plant sees little of the sky (such that it's really dark underneath), or it can mean being covered so it just sees little of the direct sun. "Full sun" plants very much don't like the former.

There is a site somewhere that defines bright shade, full shade, dappled, etc. I still find ambiguity in the terms but it is still helpful. I'll link to here is I can find it.

I always take full sun on a label or info on the net with a grain of salt when it comes to hot southcentral Texas. The question is, can the plant tolerate full on sun 6-8 or more full hours a day HERE in July when it's 100 degrees no cloud cover? I love my native plants that can do that! And then, there's the question, how often to water them.