官方文档上的说明如下：

       PURGEOLDEXTRACTS：Purges trail data that is no longer needed（按规则清除主端或者是备端的不再需要的跟踪文件）
                  
         Using PURGEOLDEXTRACTS  as a Manager parameter is preferred over using the Extract or
         Replicat version of  PURGEOLDEXTRACTS . As a Manager parameter,  PURGEOLDEXTRACTS  allows
         you to manage trail files in a centralized fashion and take into account multiple processes.
         
          Use USECHECKPOINTS  to purge when all processes are finished with a file as indicated by
         checkpoints. This is the default, but it can be turned off with the NOUSECHECKPOINTS 
         option. Basing the purging on  checkpoints ensures that no data is deleted until all
         processes are finished with it.  USECHECKPOINTS is checked whether or not the parameter
         is explicitly defined with  PURGEOLDEXTRACTS , unless there is an explicit
         NOUSECHECKPOINTS  entry. Basing purges on checkpoi nts is essential in a production
         environment to ensure data integrity. USECHECKPOINTS  considers the checkpoints of both
         Extract and Replicat before purging.

         Use the  MINKEEP rules to set a minimum amount of time to keep unmodified data:
         Use MINKEEPHOURS or  MINKEEPDAYS  to keep data for    hours or days.
         Use MINKEEPFILES to keep at least   trail files including the active file. The default
         is 1 .

           Use only one  of the MINKEEP options. If more th an one is used, Oracle GoldenGate selects one of them based on the following:
          If both MINKEEPHOURS and  MINKEEPDAYS  are specified, only the last one is accepted, and the other will be ignored.
          If either MINKEEPHOURS or  MINKEEPDAYS  is used with MINKEEPFILES , then  MINKEEPHOURS  or MINKEEPDAYS  is accepted, and  MINKEEPFILES is ignored.

简译： 使用跟踪文件中的检查点来判断是否要被清除，但是在清除时还有遵循一定的 原则。用MINKEEP 来确定要保留的最小时间。主要作用来防止硬盘空间被无限的使用。