Ven Vijitananda said it is called Rupavacara because senses are not contaminated with five hindrances.
For instance in Kamavacara the senses are contaminated with five hindrances.
Arupavacara is when you do not create a rupa in a particular relam.
For instance (my example) you can see the earth without recognising countries.
or you can recognise space without the planets.
As far as I am concern this is the best answer to my question so far.
General discussion of issues related to Theravada Meditation, e.g. meditation postures, developing a regular sitting practice, skillfully relating to difficulties and hindrances, etc.