This Wiki article will explain how to hide Middle filed for documents in edit mode.
1) Open Layout Editor and navigate to Documents items.
2) Select Edit View under the Documents (by selecting Edit from the top item group, changes will be implemented to all templates under Documents item, except the ones which were modified previously ) and go to “Properties” tab.
4) Open "Property Editor" by double clicking on property “Number” and change "Data Type" from eBRel:DocumentNumber to eB:ReadOnlyString
6) Save layout
7) Restart IIS services for changes to take effect.