The Regulations make a distinction between the permanent cover on a piece of furniture and a loose or stretch cover. The permanent cover can be defined as the cover fitted to the furniture when it is supplied to the customer even if it is removable for cleaning or changing. Similarly, replacement covers which are supplied by the manufacturer of the furniture at a later date should be regarded as permanent covers.
The key point is that the removable cover forms only the wear/decorative cover used on the furniture and is designed by the producer/supplier of the furniture to be specific to that piece of furniture. Removable covers which are supplied with the furniture or at a later date by the producer/supplier as a replacement are regarded as permanent and must comply with Schedules 4 and 5 of the Regulations.
Loose or stretch covers, on the other hand, are removable covers which are supplied by a producer other than the manufacturer of the furniture. These are designed to be placed on top of an existing finished piece of furniture which is already fitted with a permanent cover. This distinction is made because only the manufacturer of the furniture concerned will have detailed knowledge of how it complies with the Regulations.
Loose covers for upholstered furniture must comply with Schedule 5, Part 1 of the Regulations and stretch covers must comply with Schedule 5, Part 2 of the Regulations as these are sold separately from the furniture. They also need to have a permanent label attached to them and this label may appear on the underside of the fabric. There are no display label requirements for loose and stretch covers.
Check out http://www.legislation.gov.uk/uksi/1988/1324/schedule/5/made and Fire safety of furniture and furnishings in the home - A Guide to the UK Regulations guide 2.