Names in Arnor are a sort of trust. A Sindarin name is particularly to be guarded, for none know the origins of those folk who guard Eriador, save they themselves. For this reason, Sindarin names are rarely used when the Rangers are abroad in the wildwood, in mixed company. The only place where the use of these names is truly safe is in Imladris, and the other elven enclaves.
Because of this necessity, bynames are given to the Rangers for use in the wild or among strangers. Aragorn, for instance, is known outside of the Rangers as Strider. A byname is generally a reflection of the Ranger who bears it, and can be given by any of his kindred. Until he is given a byname, a Ranger will often contrive a name to be used in the wilds or in mixed company, for his Sindarin name must be guarded always. Hanneth, for example, often goes by 'Anna' and Saidonnen by 'Daisie'. Simple names that could easily be of Breefolk origin.
Titles are likewise guarded. Aragorn can be addressed as Chieftain, but never as "my Lord" or "my King." Likewise, his elven name Estel is never used by the Rangers in any circumstances. Halbarad can be addressed as Captain; other Rangers by their bynames or self-chosen pseudonyms, or simply as "Ranger." Again, this an issue of safety and protection in the wildwood, and one strictly observed by all who follow Aragorn.