roster vs. address book (or book) in Ekiga

[Александър Шопов <ash contact bg>]

Hi everyone,
The interface of Ekiga uses two expressions for things that are very
similar (at least to me) - lists of contact addresses:
One is "roster", the other one is "address book" (or simply "book"). Are
these fundamentally the same from end user's perspective and the
developers use the terms as synonyms or is there an important difference
that I am overlooking. Thus I wonder whether I should a single term for
translating these two or I should use two different ones (and I can).

Kind regards:
al_shopov