Species cannot be assigned the Least Concern category unless they have had their population status evaluated. That is, adequate information is needed to make a direct, or indirect, assessment of its risk of extinction based on its distribution and/or population status.
Since 2001 the category has had the acronym "LC", following the IUCN 2001 Categories & Criteria (version 3.1). However, around 20% of Least Concern taxa (3261 of 15636) in the IUCN database use the code "LR/lc", which indicates they have not been re-evaluated since 2000. Prior to 2001 "least concern" was a subcategory of the "Lower Risk" category and assigned the code "LR/lc" or (lc).
While "Least Concern" is not considered a red listed category by the IUCN, the 2006 Red List still assigns the category to 15636 taxa. The number of animal species listed in this category totals 14033 (which includes one novel species, Moss Frogs). There are also 101 animal subspecies listed and 1500 plant taxa (1410 species, 55 subspecies, and 35 varieties). There are also two animal subpopulations listed: the Australasian and Southern African subpopulations of Spiny Dogfish. No fungi or protista have the classification, though only four species in those kingdoms have been evaluated by the IUCN. Humans qualify for this category, although they have not been formally assessed by the IUCN.
Many category or ranking systems simply do not list Least Concern species. For example Australia's EPBC Act does not have a category for "not at risk" species, although such species may be found amongst the lists of "unsuccessful nominations" or "removed" (delisted) fauna and flora. There are, however, other reasons species may be found on these lists, such as taxonomic changes. The US Endangered Species Act likewise does not list species which are not at risk.