When you create a new AutoCAD file, can you see this defpoints layer? No, unless your template file has it defined. This layer will be created automatically by AutoCAD when you create dimension. Try to create a dimension, then look again in your layer list. What is this layer for?
This layer is used for definition points when you place dimensions in AutoCAD. This layer by default is set not to be plotted. Don’t mind this layer. Why worry about one layer when you can create virtually unlimited layers?
The problem is, many use this layer when creating objects they don’t want to plot. Viewports for example. This is practical before, we can’t define a no-plot layer. Now that we can create a no-plot layer, we should avoid using this. Why?
Layer defpoints is somehow linked to Layer 0. When we freeze or thaw layer 0, you can’t modify objects in layer defpoints. You probably know it, but someone who receive your file might not.
The other reason is, when you run audit, all objects that’s not definition points will be removed to a new layer: $AUDIT-BAD-LAYER. You can read the explanation here. There is no explanation whether this will make your files corrupted or not. But if audit treat it as ‘bad object’ then it must be a good reason for it. You don’t want to see the ‘deadly’ fatal error message, do you?