In compiling the specialized dictionaries it is very important to develop the conceptual foundations of designing the lexicographical sources, which are inseparable from the concept of the language for special purposes, since the terminological vocabulary and special terms appear primarily in the sphere of professional activity, which broadly provides a language for special purposes (LSP). The identification of the range of sources, as a part of the selection process of special vocabulary, is a key direction in compiling the terminological dictionaries. It depends largely on methodological arrangement of the dictionary and it is aimed at obtaining the information about the term taking into account its conceptual form, systemic and intrasystem communication of terminological system.