I used the term "pad" in a generic sense. This is a lush backing track that usually has a polyphonic score of sustained instruments such as strings, choir, organ, or synth pad. If the style engine has a chord form parameter, they will usually be voiced "close," changing as few notes as possible during chord progressions. They usually have a "dark" timbre so as not to take away from the melody or rhythmic phrases. Sometimes they also have a longer attack and release. What you want to avoid is a strong rhythmic pattern, because then it will be less versatile (locked in to a specific tempo and time signature.)

On a Roland this type of sound might be found on any of the Accompaniment (AC) tracks. Look at orchestral or ballroom styles like Slow Fox, Slow Waltz, etc. for instruments like Slow Strings, Contrabass, etc. Usually arrangers have a facility to listen to each of the accompaniment tracks in isolation. This will help you find one that's suitable.

When you've mixed a couple of tracks like this into a user style and add a subtle bass line, you end up with something very close to Yamaha's "free play" styles. Even the less expensive Casios have a style like this called "String Quartet." If you activate Sync Stop, the backing will fade away when you release the keys, and the bass will give a new attack with each chord. This is great for playing rubato, or any kind of music that doesn't follow a strict time signature (which would otherwise be very hard to play on an arranger.) Free play styles aren't magic or special programming- just a unique application of what most arrangers have been able to do all along!