When you create a new document setting, such as a security rule, it can be set as 'default'. 

Setting a document setting as default means it will be used when you create a new document. 

How to set as default
When creating a new document setting, tick the 'Default' checkbox.

Example
We have ticked 'Default' for a reader login security rule. This means this security rule will be applied when a new document is created.