The ReUse in Application Engine was brought to my attention by a great blog that I follow regularly.  The original post can be found at http://peoplesofttipster.com/2007/07/31/reuse-in-app-engines/. 

The ReUse statement setting on an Application Engine is a great way to increase performance. It’s useful when you have a SQL object that is called multiple times, for instance when you have a DoSelect that runs a SQL Step repeatedly, substituting different Bind variables in each time.

The default setting for ReUse is ‘No’, which means that each time the SQL Step is called it is recompiled and new execution plan generated, bad news if the SQL has to run a large number of times. If the ReUse setting is ‘Yes’ then any %Binds in the SQL statement are converted into ‘:1’, ‘:2’ etc. This means the statement can just be compiled once and the same plan used each time, resulting in really quite dramatic improvements in processing time.