It is not unusual for an enforcement officer to invite the submission of a retrospective planning application during the course of an enforcement investigation, i.e. an application for planning permission for development which has already taken place. Doing this normally suggests that the Local Planning Authority (LPA) might find the development acceptable, but this is not certain, for instance if an issue not anticipated comes up when the application is processed.
In some instances the LPA might consider the development acceptable subject to conditions, e.g. if the unauthorised development is a restaurant near houses, they may want to impose a condition requiring it to close by a certain time, so a retrospective application is invited to enable such conditions to be imposed.