Much of the X++ code you write can run on either the Application Object Server, or the Client. The client can be the Win32 client, or BC.NET for Enterprise Portal. MorhpX generally will not direct your code to run on a specific tier, but you as an application developer can control it.
Whether code runs on the server or on the client has performance and security implications, and should be given careful consideration as a part of design. Occasionally it will be correct to methods run on either tier, but usually this is not the case.
As MS move forward implementing tasks in upcoming release, developers will identify which resources to grant access to for a given task. These resources include tables/fields and server entry points. A server entry point is a specially decorated method that runs on the server. Tables/fields accessed inside this server entry point need NOT be identified with the task, greatly reducing the complexity and effort required to implement the task. If the server entry point is NOT executed on the server, then table/field access is still validated, and will fail if the code accesses a table/field that is not included with the task.
So, in short:
Set the RunOn AOT property to Server or Client for new classes, unless you have a reason to leave the default
Include considerations about where the code runs in your design, and look for it when reviewing design documents
Declare static class methods, table static and instance methods with either the "server" or the "client" keyword, unless you have a reason not to
Look for ways to limit direct table and field access in form methods, try to move this into a class
This page MSDN describes where methods run:
http://msdn.microsoft.com/en-us/library/aa891949.aspx. Additional information to cover more object types at http://msdn.microsoft.com/en-us/library/aa634829.aspx.