The study of languages and their variations play an important role in the formation of language teachers. It is important to foster the awareness of the student of additional language to linguistic variation relating it to the situations in which it occurs.

Analyze the following situations.

Police officers in Switzerland use the Swiss variation of German language in intimate and casual circumstances and the standard variation of German language in formal occasions. Inhabitants of urban regions of Paraguay use Spanish language to communicate in formal occasions and switch to Guarani language in intimate and informal conversation among themselves. Arabic language speakers who generally use the vernacular variation of Arabic language, make use of modern standard Arabic language in public speeches.

Those situations are examples related to which of the following concepts?