In most projects, the i2b2 ontology resides in a default database table called I2B2. If you have a large ontology, maybe consisting of concepts from different sources, it might be a good idea to split the ontology into multiple tables. That will simply simplify maintenance as you need to update only the table where the ontology has changed.
The table TABLE_ACCESS contains references to all other tables that are part of the ontology. See the example Boston demo database: