There are three types of allocation units.

IN_ROW_DATA (default): If a table is relatively simple in desin (meaning record size is less than 8000 and no LOB data types are present), all records are stored in a pages refered as IN_ROW_DATA pages.

ROW_OVERFLOW_DATA: assume that a table is created with record size 12000 bytes having 4 varchar data types of size 4000 bytes. Whenever user inserts a record with size greater than 8000 (page size is 8K), then the excess data is moved to ROW_OVERFLOW_DATA pages. In simple terms, ROW_OVERFLOW_DATA pages will come in to picture only when the row size exceed page maximum limit.

LOB_DATA: LOB data like text not stored along with data pages. LOB data is stored in pecial pages called LOB_DATA pages. 16 byte pointer in data page will be used to refer LOB_DATA page.