Available names which cannot be interpreted and are not used, e.g. if the type is missing, can be a problem for taxonomists. Such a name is often referred to as a nomen dubium (plural nomina dubia) or dubious name.
Such names do not become unavailable, even if they are not used as valid, they may become valid again in the future as they can continue to compete in homonymy and, if later applied to taxa, can compete in synonymy. This may be a problem for the stability of names e.g. as potential senior synonyms or homonyms may overturn the established usage of junior names.
The Code can allow prevailing usage to be kept where this will aid stability, even where this goes against the principle of priority. E.g. a forgotten name not used as valid since 1899, and found to be a senior synonym or homonym, can be declared a “nomen oblitum” and an established junior name for the same species a “nomen protectum”. This makes the junior name valid and the senior name invalid (Articles 23.2, 23.9.2). Difficult cases can be referred to the Commission for a ruling.