They're all mostly true, although a central consideration is that a directory becomes a meta-directory when it's used as the focal point to manage other directories. Not just any data store can be considered a meta-directory. It must have at least the capability to obtain information from other data stores through standard or proprietary means, and then display that information to users and applications through its own interface. Beyond this characteristic, a meta-directory is largely an implementation issue rather than a product issue. In most situations, they'll probably both be used in combination to provide a logically unified directory service.
Synchronization is done through the replication of information from one data store to another.
This is a necessary feature of an independent directory service, such as NDS or AD, in order to maintain accurate and authoritative information within the distributed databases that make up the respective directories. In many cases, a directory can be the master of some information areas and a slave to other areas. In such cases, a directory holds a peer status between participating directories. Chaining, which is also referred to as brokering, is the retrieval and display of information from another data store without actually updating the local data store. It's essentially a data request.