My experience: rayon (or synonym viscose) is a cellulose fibre, like organic cotton. unlike organic cotton the fibres are created artificially, e.g. from wood. because created articially, rayon fibres are longer and more consistent than organic cotton (or hemp or bamboo).
in my experience: rayon transports liquid much faster. mostly irrelevant for RDAs, relevant for many RTAs.
the capability to store liquid can be less than in organic cotton. rayon tends to be more fluffy than than organic cotton.
in my experience rayon can has much less resistance against overheat. on dry hits, rayon tastes much worse than organic cotton. completely dry it burns faster than organic cotton.
depending on how you vape, coil and wick it can be a gain to use rayon (faster liquid transport). rayon does have no taste, unlike some organic cottons.
but if your coils has hot spots or if you vape at high wattages/high temperatures, rayon might be the wrong for you.
if you vape at more moderate wattages, and if you have an RTA with leaking/flooding/gurgling probs, sometimes rayon can fix that. because of the faster liquid transport there is less danger of flooding/gurgling. not zero danger, but less.
at the small german town where i life, in the local drug store i can buy rayon cutton (sold for removing make up and so on). works fine for me as long as i do not overheat my coils. you do not need to buy special vapers rayon cotton to test if rayon is good for your wicks, or not.