When you make a container you place borders around pieces of emptiness. The emptiness is given shape and becomes a space inside the border.
This space is often used to protect or separate matter of different value from each other or to sort the world to make it easier for us to use. For instance keeping the sugar from being mixed with all the other things on the table until we decide the time has come to mix them, or separating the bones of a saint from all mundane materials inside a reliquary, or building walls to make sure people stay in there assigned places.
In short: to protect what is contained against unwanted influences or to contain and isolate what is unwanted.
The border (the material divider) can itself be used to tell you something of what it encloses and how important it is, and also if it is closing something in or closing something out. But can it also contain the movement between the inside and the outside?