What's the difference between avatar and Splinter?
These are all very weird terms that I'm just using.
*mistakenly answering for Sliver* A Sliver is a person who has held the power of a Shard, and then let go of it. A briefly held time, holding the infinite power of a Shard, but no longer does. So what does that do? That changes your soul, and leaves markers on it. It's a real physiological thing.
An avatar is... a Shard manifesting a semi-autonomous piece of themselves that is still connected to who they are. An avatar, for instance, of Autonomy - depending on how Autonomy creates that avatar - might know, might not know, but they are still an aspect, they are still part of Autonomy. And when you get down to it a part of them knows that, and it's almost a god roleplaying, but in a way that only a Shard, or a lowercase-g god in the Cosmere, can do.
*realizes that he answered for Sliver earlier, and clarifies*
A Splinter is a piece of a Shard that is fully autonomous, where an avatar is not. So something that is Splintered does not consider itself - and would not be considered by the definitions - an actual piece of it [the Shard], and has free will. So once it has free will, and/or could develop free will (because some of the Splinters haven't gotten there yet), but is fully cut off from the direct control and self-identity of the Shard, then it is called a Splinter.