Names of subspecies are governed by the Code and are available names (if they meet the criteria for proper publication). A scientific name added as a trinomen on the end of a bionomen is taken to indicate a subspecies.
However, it stops there. The ICZN does not govern names for groups below the level of subspecies (infrasubspecific groups), as is likely to be the case for populations. This also means that the Code does not regulate names of groups of specimens that differ because of intrapopulational variation such as differents sexes, different castes, age groups, seasonal forms, different generations or variants that are part of a non-interrupted polymorphism.
How can you tell if a name is intended to indicate a subspecies or a level below a subspecies? If the author adds a name on to a trinomen, then this indicates infrasubspecific rank. If the author added a trinomen (which would normally indicate a subspecies) but also used the term ‘abberation’, ‘ab.’, ‘morph’, ‘variety’, ‘var.’, ‘v.’, ‘form’, or ‘f.’ (with some specifics on dates for each of these terms, see Art. 45.6), this also indicates the entity is an infrasubspecific group, thus the name is not available according to the Code.