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