Entity Usage
When viewed in Table Mode (see Editing a Wind Document for details), all entities do all have a column with the label "Used" as title.
The location of the Used column can vary from one entity table to another - but its purpose is always the same: tell you if and where a given entity is used by another entity.
The USED button presents two states: 
grayed out (inactive) and 
green (active).
When you move your mouse over an active USED button, a yellow tooltip opens and lists all occurences of the selected item:
Items which are used by other WIND entities (referenced in other documents) can not be removed. You will need to spot all the places where there are used (this is where the usage list is very useful) and remove all references to the item first. Only when this is done, and the Used status returned to inactive, can you finally delete the entry.