With adaptive SQL plan management, DBAs no longer have to manually run the verification or evolve process for non-accepted plans. When automatic SQL tuning is in COMPREHENSIVE mode, it runs a verification or evolve process for all SQL statements that have non-accepted plans during the nightly maintenance window. If the non-accepted plan performs better than the existing accepted plan (or plans) in the SQL plan baseline, then the plan is automatically accepted and becomes usable by the optimizer. After the verification is complete, a persistent report is generated detailing how the non-accepted plan performs compared to the accepted plan performance. Because the evolve process is now an AUTOTASK, DBAs can also schedule their own evolve job at end time.
Note:
*The optimizer is able to adapt plans on the fly by predetermining multiple subplans for portions of the plan.
*Adaptive plans, introduced in Oracle Database 12c, enable the optimizer to defer the final plan decisionfor a statement until execution time. The optimizer instruments its chosen plan (the default plan) withstatistics collectors so that it can detect at runtime, if its cardinality estimates differ greatly from theactual number of rows seen by the operations in the plan. If there is a significant difference, then theplan or a portion of it will be automatically adapted to avoid suboptimal performance on the firstexecution of a SQL statement.

Explanation
With adaptive SQL plan management, DBAs no longer have to manually run the verification or evolve process for non-accepted plans. When automatic SQL tuning is in COMPREHENSIVE mode, it runs a verification or evolve process for all SQL statements that have non-accepted plans during the nightly maintenance window. If the non-accepted plan performs better than the existing accepted plan (or plans) in the SQL plan baseline, then the plan is automatically accepted and becomes usable by the optimizer. After the verification is complete, a persistent report is generated detailing how the non-accepted plan performs compared to the accepted plan performance. Because the evolve process is now an AUTOTASK, DBAs can also schedule their own evolve job at end time.
Note:
* The optimizer is able to adapt plans on the fly by predetermining multiple subplans for portions of the plan.
* Adaptive plans, introduced in Oracle Database 12c, enable the optimizer to defer the final plan decision for a statement until execution time. The optimizer instruments its chosen plan (the default plan) with statistics collectors so that it can detect at runtime, if its cardinality estimates differ greatly from the actual number of rows seen by the operations in the plan. If there is a significant difference, then the plan or a portion of it will be automatically adapted to avoid suboptimal performance on the first execution of a SQL statement.

(http://docs.oracle.com/database/121/ADMIN/schema.htm#ADMIN11601)
The Segment Advisor generates the following types of advice:
If the Segment Advisor determines that an object has a significant amount of free space, it
recommends online segment shrink. If the object is a table that is not eligible for shrinking, as in
the case of a table in a tablespace without automatic segment space management, the Segment
Advisor recommends online table redefinition.
If the Segment Advisor determines that a table could benefit from compression with the advanced
row compression method, it makes a recommendation to that effect. (Automatic Segment Advisor
only. See "Automatic Segment Advisor".)
If the Segment Advisor encounters a table with row chaining above a certain threshold, it records
that fact that the table has an excess of chained rows.

Explanation
http://docs.oracle.com/database/121/ADMIN/cdb_pdb_admin.htm#ADMIN14017