Cats scratch objects for the following reasons:
- to remove the outer sheath of the claw from the front feet. This reveals the sharper fresh claw underneath.
- to exercise the mechanism made up of tendons, bone and muscles, that protracts the claw when required e.g. attacking and grabbing prey. The claw is normally retracted.
- to stretch the back muscles (this is probably incidental to the primary functions).
- as a means to communicate in two ways: (1) depositing scent from glands in paws - olfactory communication and (2) leaving scratch marks - visual communication.
If in a multi-cat household a cat or cats mark territory through scratching and/or spraying or depositing feces it is probably due to stressful social interactions. Multi-cat households might require some management if the cats where selected to join the group by the person and not by the cats as is usually the case (in feral cat colonies). That might sound odd but cats have preferences. They like and dislike other cats and people.
A scratching post is an important piece of cat furniture. However most commercially available models are too small. The post should be of a sufficient height to allow the cat to fully stretch upwards. Scratching on armchairs occurs towards the arm rest or higher at the back of the chair; that sort of distance off the ground. We are looking at about three feet tall or more. Cats will also scratch horizontally (e.g. on carpets) so perhaps two devices should be installed, both of solid construction.
Never "solve" this so called cat behavioural problem by declawing the cat. That is cruel and unnecessary. It also tends to create other problems that cannot be resolved. There are much better, gentler and more moral alternatives.
Associated: Will my cat use the scratching post?