Append and Append To basically deal with the entities that are parties to a 1:N relationship or N:1 relationship.
Append: When an entity has the lookup of another entity on its form. It is important that the user have the “Append” privilege on this entity so that it can set the values for the lookups on this entity. For eg: Contact has the lookup of Account on its form so here the user needs to have the “Append” privilege to be able to set the parent account.
Append To: When an entity is available as a lookup on another entity form. It is important that the user have the “Append to” privilege on the entity that is referred to in the lookup so that it can set the values for the lookups of this entity on any other form. For eg: Account has the lookup of primary contact. So here the user needs to have the “Append To” privilege to be able to set the Primary Contact for the Account.
Lets us take an example of Contact entity.
We have modified the Order Role and removed the “append” privilege to the Contact entity.
Now we have modified the Order Role and provided the “Append” privilege for the contact entity and removed the “Append To” privilege from the contact entity.The Primary Contact on Account is disabled.