There are two types of external sets: primary and reference. A primary OSM (EXT) provides a permanent storage space for your documents that is both readable and writable. Once documents leave your cache OSM, if your primary set has a write cache, they move to the permanent storage provided by the external OSM set. A reference OSM is used during migration and allows read-only access to objects stored in an extant storage solution. You should create OSM reference sets if performing an in-place migration and have an import solution.