not in is used to check if a value is present in a list or a taxonomy. If it is not, the expression evaluates to true.
On the left, there is always a single value. The list of options, or the taxonomy, goes on the right side.
If a document is only relevant to companies who are not registererd in the European Union, attach this condition to it:
e is Entitye.domicile not in EUCountries
There is one answer for what the domicile of an entity is, so this is a single value. With the conditions in the example, you check whether this value is not a part of the taxonomy
EUCountries, which predictably consists of a list of all the member states of the European Union.
e.domicile is in the United Kingdom, the condition evaluates to true. The document will be included in the case. If
e.domicile is Belgium, the condition evaluates to false, meaning the document is excluded.