poki are containers. They're meant to contain things. Bags, boxes, and bins are great examples of poki. Furniture that contains things, such as trash cans, drawers, shelves, and even closets, are a big part of poki. Some poki might be metaphysical rather than physical. All semantic spaces of all toki pona words can be framed as poki, containing possible meanings. You could look at labels through this lense, and people who choose to use a label are placing themselves within the semantic space of said label. This usage of poki is fairly common.
tomo - tomo is defined by having a specific relationship to its contents. The thing that is inside of it must be using it as a space to reside, a space to live, a space to perform activities. Using tomo to describe something implies this. poki, on the other hand, lacks these connotations. Any tomo could be described as a poki, but it would remove all of those connotations which is often undesirable. poki instead just connotes holding things within.
supa - supa are flat and face upwards. That is the only important aspect of their semantic space. Most of the supa we encounter in our lives have things on them: either us, or our things. We cook on supa, we work on supa, we sleep on supa. Almost everything we do is in a supa. These are all implied by the core meaning of a flat surface. By using poki to describe something usually described as a supa, we are changing the relationship from "an upwards facing flat space, perfect for things to sit atop" to "an object meant for containing other objects." The semantic spaces might be similar, but they differ in these connotative patterns.
lupa - In order to be a lupa, something else must be able to go inside it, either to stay or just to pass through. That doesn't mean that is what it's for. Many lupa can be used as a poki of some sort, but lupa does not connate contents. poki does. A hole I dig in the ground is a lupa, but if I lower a coffin into it, it becomes a poki. Some lupa cannot be poki, such as a glass window, because they cannot contain anything and only serve to allow things to pass through.
To return to the dictionary, click here.