Hello;

What is usage of $BBX,$BBY,$BBZ? (and $BBX2,$BBY2,$BBZ2)?
Can you give me some practical examples?

Thanks.

They are (pretty much, as I understand) deprecated these days, and have a modern equivalent in vex (relbbox() ).

But basically, there are a lot of uses for the current position within the bounding box. For example: if you want to color something along its height, but independent of world size (for example: no matter how big the cake is, the top 10% should always be covered in sugar coating). For that you need to know where the current point is in relation to the bounding box.