It is important to distinguish between typical sensual attraction to friends and romantic partners, and demisensuality, which implies that there would be no sensual attraction to anyone unless a close relationship was to be established. The already existing relationship is the only way a demisensual person would be able to experience sensual attraction. Being sensually attracted to one's close friends does not necessarily mean that one is demisensual, for example.
Demisensual can be a sensual orientation on its own or can be combined with other orientations. For example, one could be demisensual and bisensual (demibisensual), meaning that when one does experience sensual attraction one can experience it toward two or more genders.
The prefix demi- means half or partial.