Target load plan is that you can set at the Informatica 
level whereas the Constraint based load ordering is what 
happens at the database side.

Whenever you have multiple pipelines in your mapping, you 
can set the order of execution of those pipelines, which to 
run first and which next. Whenever u select the target load 
plan, it displays the list of source qualifiers lying in 
the different pipelines. You just need to arrange the 
source qualifier as per your requirement and save your 
mapping.

Constraint based load ordering is an option in the session 
properties where the informatica understands that the 
parent table has to be loaded before the child table. You 
need not determine which table to load first and which 
next, all you need to do is just check the 'Constraint 
based load ordering' option in the session properties. This 
applies when you have multiple targets in the same pipeline 
simultaneously(like multiple targets from the same 
transformation or multiple targets from the same output 
group of a router etc..)