From Félix-Brasdefer (2015, The Language of Service Encounters [Ch. 8])
Forms of address are devices that refer to the addressee and are studied under person and social deixis. Person deixis concerns the identification of the addressee or participant roles and is commonly realized through personal pronouns and vocatives (e.g. titles, kinship terms, and proper names). Social deixis is expressed through the codification of the social status between speaker and addressee(s) or speaker and some referent (Levinson 1983: 63). In English and Spanish, as in many other languages, social distinctions between the relative ranks of speaker and addressee are systematically encoded to express familiarity, formality, and respect.
Unlike modern English which has one pronominal form ‘you’ to address an interlocutor in both formal and informal contexts, Spanish uses two pronominal forms, an informal pronoun tú/vos (‘you-informal’) (T) and the formal form usted (‘you-formal’) (V). In other varieties of Spanish the T/V distinction adopts different forms: the informal plural forms include ustedes (mainly used in Latin America) and vosotros (spoken in most regions of Spain, but absent in the Canary Islands and America), while the singular T form vos is present in many regions of America, but absent in Spain and the Canary islands (Fontanella de Weinberg 1999). And yet in other varieties of Central and South American Spanish, the pronominal forms usted and vos adopt singular interpersonal functions to express intimacy, deference, and politeness with power equals (e.g. friends, family) (e.g. Costa Rica [Moser 2006] and in other regions of Central and South America [see readings in Hummel, Kluge, and Vázquez Laslop 2010]).