i thin it is The diagonal
You could, but outdoor arenas are often called menages, an indoor arena can be called a indoor school, or an indoor arena.
in an indoor riding school you can not do a perpendicular
The perpendicular
An indoor horse arena is a controlled environment where horses can be ridden or trained regardless of weather conditions. It typically consists of a covered area with footing designed specifically for horses and may include seating for spectators. Indoor arenas are commonly used for various equestrian activities such as riding lessons, competitions, and exercise.
It only depends on how small the indoor is. If it is large you can do any figure in an outdoor ring.
The perpendicular.
National Indoor Arena is an indoor arena in United Kingdom and was opened on 1991.
It is an 18,000 seat indoor arena...
Podium Arena is an indoor arena in Poland and is is under construction.
While riding in at a riding school either in an indoor or outdoor arena people are expected to pass left to left. Anyone going at a faster gait has the right of way. Slower riders should move towards the inside track and not ride on the rail.
Arena Armeec is an indoor arena in Bulgaria and was opened on 2011.
SAP Arena is an indoor arena in Germany and was opened on 2005.