When we click a share button, a universal "Share Target" pane slides up. Apps declare Share Targets, which is a way that the user can share something from one app to another. This is how, say, we can share a tweet on facebook. It gets even more complex now that share targets can be created for specific contacts. So we can directly share an item with a specific person, through a specific app.
In the developer docs, you can read about how developers can implement certain features to try to increase the "ranking" of their share targets. Basically, they want to get their share buttons closer to the front than share targets from other apps. You can also directly rank the share targets within your own app. Android has some sort of algorithm that dynamically determines the rank of share targets between apps, and this ranking changes with the user's behavior.
What I want to do, however, is outside of the scope of that documentation. I want to "pin" a certain share target, so that is always the very first share target displayed regardless how much I use it. And I only want to do this on my device, so I am not trying in anyway to get around android's ranking system to improve the ranking of my app on other people's devices.
It just so happens the app whose share target I want to "pin" in one I am developing myself, but I suppose such a feature to manually control the ranking of share targets on your own device could be useful regardless whether or not you are developing your own app.