I'm writing a piece for a magazine

[Bacchus]

Can any owner of a velomobile help me?

Not an owner here (I ride an open recumbent) but I have experience in them.

- Those bicycles are used in competitions, right? Besides that, do you use it in a regular way to go to work or to commute?

Velomobiles are primarily built as practical human powered vehicles. These are multiple track, often tricycles. That makes them more stable and a bit more roomy and practical, which are beneficial properties in a public road environment. Of course, like basically all vhicles they are raced too but they definately are not purpose-built race machines.
Next to velomobiles, there are faired recumbent bicycles. These are only built for competition and record attempts and are not safe to ride on public roads. They are the fastest human powered vehicles on land though with a 200 m sprint record of 144 km/h (Reichert) and 92 km hour record (Russo). The BHPC sees a few in their track competition.

- and if there is any norm, rule, or law under which they have to be used?

There are no specific laws. Generally they are considered to be bicycles except some can be considered multi-track cargo bicyles by their width of over 75 cm and thus are allowed on a main road (but not motor way) where a parallel bicycle path is available.