Abstract In this document, we classify concepts, terminology and notations with respect to their usability in IOI competition tasks. We distinguish three usability classes: basic knowledge (can be used without further ado), to-be-defined (can be used, but must be defined explicitly), and to-be-avoided (must not be used). The classification primarily concerns the use in task descriptions, which are presented to contestants at the beginning of the competition. But it also concerns solutions handed out after the competition. This classification can benefit both organizers and participants in their preperations for an IOI competition.