Pools allow you to store content that should be separated logically. If all of your sites should import and export the same content, you can simply create one pool and set it to "Force" everwhere.
Otherwise, after having enabled the export or import for an entity type bundle, you can configure if and how the entity should be exported/imported to each pool.
Export - Forbid
Entities of this bundle will not be exported to this pool at all.
Export - Force
Entities will automatically be exported to this pool.
Export - Allow
The content managers are able to select to which pools an entity should be exported to. This is especially useful for Content Syndication.
Import - Forbid
No entities for this bundle are imported from this pool.
Import - Force
All entities will automatically be imported from this pool.
Import - Allow
Import entities of this bundle from this pool only by using the Manual Import Dashboard.