It is very important for DV to know the correct state of all the advertisements. For every object it needs to know if it is advertised or not. And, if not advertised, the reason(s) why. DV uses this information to decide what to do next (and when). If action from the advertiser is required (e.g. complement the object information) DV makes this information available to the advertiser (see II.4. Feedback).
For every mutation DV sends to the site (add, update, remove) DV expects an answer including at least if the mutation has been processed successfully (yes or no) and if not processed successfully, the reason(s) why. It is possible to have delayed feedback, for instance if the mutation has been put in a queue for further processing.
If for some reason the wrong feedback is returned, for example if the site responds with negative or incorrect feedback, but the request has been processed anyway or vice versa, this can result in adverse effects for the advertiser, like duplicate, orphaned or missing advertisements.
Besides feedback directly related to requests originating from DV there are also scenarios where the site needs to inform DV of changes that originated outside DV. For example if the quality control department of the site manually removed an advertisement.
If the feedback did not result in a positive outcome, DV considers the action (add, update, remove) failed and will automatically retry at a later time. The delay depends on the type of error(s). DV will only stop retrying once the action has successfully been processed or if the action is no longer needed. For example, if the object has been sold and no longer needs to be added.