Electoral Registration Officers (EROs) sometimes have information to indicate that a person is resident at a given address but is not yet registered to vote at that address. Such people are pending or potential electors.
The information indicating that an elector is resident at an address could have come from various sources. For example, it may have been provided by the potential elector themselves, if they have applied to register to vote but their application has not yet been determined. Alternatively, their name might have been added to a Household Enquiry Form returned to the ERO as part of the annual canvass. Or the ERO may have obtained information from local data sources, such as council tax records. Individual citizens can only be added to the register once they have completed an application which allows the ERO to determine their eligibility.