In all cases, any existing relationships on the other end are preserved unless you create a filter as described in the plugin description under overwrite settings.
Whether or not the new relationship is added depends on the number of selections that are allowed in the field for the post you are creating the relationship to.
A post object field by default only allows 1 selection. If the post you are creating the relationship to already has a selection the new one will not be added unless you create an overwrite filter. If you do create an overwrite filter in this case then the old relationship will be deleted and the new one will be created. However, if you set the post object field to allow multiple selections then then an overwrite filter is not required and this plugin will add the new relationship and preserve any existing relationships.
A relationship field by default allows an unlimited number of selections. If the post you’re creating the relationship to has this type of field then the new relationship will be created and all other relationships preserved. However, if you have set a maximum number of posts that can be selected in that field and the post you’re attempting to create the relationship to is already at the max then the new relationship will not be created unless you have created an overwrite filter for that field.
