A partition type DB is a DB configured to be a Child database of the Gate. This means that every object that will be defined in the Gate, will be reflected in the partition (That property is configurable under the sync mode, to learn more click here). Objects may be table schema, triggers, synonyms, views and stored procedures.
Every partition DB has a PartitionId, which is the identifier upon which the Gate knows how to direct data and actions, to and from.
A partition may be fully matched with the gate, or partially only, depends on the needs and requirements of the business. It is possible for a single partition type DB to hold a different set of objects than the Gate.
One important concept when talking about partition and partitionID is the union option. Setting a configuration as a union is done at the Gate level. Union means that the partition column is unique across all partitions, while setting union to false, means that the partitionID is unique only in the boundaries of a specific partition.