> Antoine Cailliau <mailto:antoinecailliau@gmail.com>
> 3. Mai 2016 um 08:58
>
>     The \definefontfamily command is limited to the default
>     alternatives (tf, bf, it, sl, bi, bs ans sc).
>
>
> Thanks for your answer, what shall I use to define non-default 
> alternatives?
At the moment your only option is to write your own typescript for the 
fonts.

\definefontalternative[sb]
\definefontalternative[si]

\starttypescript [sans] [source-sans]
   \definebodyfont [default] [ss]
     [tf=Sans             sa 1,
      it=SansItalic       sa 1,
      sb=SansMedium       sa 1,
      si=SansMediumItalic sa 1,
      bf=SansBold         sa 1,
      bi=SansBoldItalic   sa 1]
\stoptypescript

\starttypescript [sans] [source-sans]
     \definefontsynonym [Sans]             [file:SourceSansPro-Regular.otf]
     \definefontsynonym [SansItalic]       [file:SourceSansPro-It.otf]
     \definefontsynonym [SansMedium]       [file:SourceSansPro-Semibold.otf]
     \definefontsynonym [SansMediumItalic] 
[file:SourceSansPro-SemiboldIt.otf]
     \definefontsynonym [SansBold]         [file:SourceSansPro-Bold.otf]
     \definefontsynonym [SansBoldItalic]   [file:SourceSansPro-BoldIt.otf]
\stoptypescript

\definetypeface [source] [ss] [sans] [source-sans] [default] 
[features=default]
\definetypeface [source] [mm] [math] [xits]        [default]

\setupbodyfont[source]

\starttext

Upright {\it Italic} {\sb SemiBold} {\si SemiBoldItalic} {\bf Bold} {\bi 
BoldItalic}

\stoptext

Wolfgang