2.4 Scope Index File Format
A scope index file stores a set of scope index records. Each scope index record is associated with a unique scope index key and stores document identifiers (1) for all items that belong to a specific set pointed to by the scope index key.
The set can include, for example, all items on a particular site, all items authored by a particular person, or all items that have a given extension, and can be used to limit the items returned by a search query.
A compound scope index is a scope index file that MUST contain zero or more records with compound scope index keys and one record with a max key.
Scope index records MUST be ordered by scope index key in default index key sorted order.