Here’s an easy example to help remember the distinction between the Append and Append To Rights defined in CRM's Security Roles.
Consider the situation when a user needs to set the Parent Customer field on the Contact. Beyond the obvious ‘write’ permission on the contact, they also need the “Append” right for that Contact and the “Append To” right for the intended parent Account record.
For more information, Joel Lindstrom provided an explanation in this blog post. (http://blog.customereffective.com/blog/2009/11/microsoft-crm-append-vs-append-towhats-the-difference.html)