"The T-SITA proposes a non-exhaustive, semantically structured, list of traits and ecological preferences. Terms are organized into a conceptual hierarchical tree with their fathers and sons ('Browse hierarchy' tab). Each term is conceptually included into its father term. For instance, the 'Reproduction type' trait term was included in the father term 'Physiology'. It means that the concept linked to the 'Reproduction type' term is included into the concept linked to the 'Physiology' term. 'Reproduction type' has two son terms: 'Asexual reproduction' and 'Sexual reproduction'. It means that the concepts linked to the 'Asexual reproduction' and 'Sexual reproduction' terms are included into the concept linked to the 'Reproduction type' term.
Each term is typified by a unit. Traits and ecological preferences terms are identified by having either a numerical unit or being 'categorical'. Quantitative traits/ecological preferences are informed by numerical values and have therefore a numerical unit. For instance, the unit of the 'body length' trait term is mm. Otherwise, qualitative traits/ecological preferences are informed by textual data. To be usable, they need to be categorized into attributes, e.g. by fuzzy coding procedures (Chevenet et al. 1994). As a consequence, their unit is specified as being 'categorical'. For instance, the unit of the 'habitat' preference term is 'categorical'. 'Habitat' is categorized into several attributes, such as 'Agricultural area' or 'Wetland' terms. Such attributes appear as son terms of the habitat preference in the thesaurus hierarchy. Attributes are identified in the thesaurus by having a unit specified as being an attribute. For instance, the unit of 'Wetland' term is 'attribute'. In addition, a categorical trait or ecological preference can have multi-levels sons. For instance, the habitat preference is the father of 'Agricultural area' which is itself the father of 'Arable land', 'Fallow' and 'Perennial crop'. It was done to take into account the heterogeneous accuracy of the textual literature informing the categorical traits or preferences."