Keyword: shows the difference.
*Full ADDM analysis across two AWR snapshot periods Detects causes, measure effects, then correlates them Causes: workload changes, configuration changes Effects: regressed SQL, reach resource limits (CPU, I/O, memory, interconnect) Makes actionable recommendations along with quantified impact *Identify what changed /Configuration changes, workload changes *Performance degradation of the database occurs when your database was performing optimally in the past, such as 6 months ago, but has gradually degraded to a point where it becomes noticeable to the users. The Automatic Workload Repository (AWR) Compare Periods report enables you to compare database performance between two periods of time.
While an AWR report shows AWR data between two snapshots (or two points in time), the AWR Compare Periods report shows the difference (ABE) between two periods (or two AWR reports with a total of four snapshots). Using the AWR Compare Periods report helps you to identify detailed performance attributes and configuration settings that differ between two time periods.
Reference:Resolving Performance Degradation Over Time

http://docs.oracle.com/database/121/ADFNS/adfns_design.htm#ADFNS1424

*Automatic segment space management (ASSM) is a simpler and more efficient way of managing space within a segment. It completely eliminates any need to specify and tune the pctused,freelists, and freelist groups storage parameters for schema objects created in the tablespace. If any of these attributes are specified, they are ignored. *Oracle introduced Automatic Segment Storage Management (ASSM) as a replacement for traditional freelists management which used one-way linked-lists to manage free blocks with tables and indexes.
ASSM is commonly called "bitmap freelists" because that is how Oracle implement the internal data structures for free block management.
Note:
* Buffer busy waits are most commonly associated with segment header contention onside the data buffer pool (db_cache_size, etc.).
*The most common remedies for high buffer busy waits include database writer (DBWR) contention tuning, adding freelists (or ASSM), and adding missing indexes.

Oracle Data Pump disable redo logging when loading data into tables and when creating indexes. The new TRANSFORM option introduced in data pumps import provides the flexibility to turn off the redo generation for the objects during the course of import. The Master Table is used to track the detailed progress information of a Data Pump job. The Master Table is created in the schema of the current user running the Pump Dump export or import, and it keeps tracks of lots of detailed information.