Extension shelf

An add-on shelving unit is a shelving unit that cannot stand on its own and must always be attached to an existing basic shelving unit. Each additional bay of a row of shelving is referred to as an add-on shelf. It is always necessary to start with a basic shelving unit in order to add a new shelving unit.

Is it possible to use an add-on shelf separately without a base shelf?
No, an add-on shelf does not have a second side upright, which would be necessary for stability. Inexpensive add-on shelving always requires a base shelf to which the crossbars and shelves of the upright shared with it are attached.