This question replaces my previous, unclear, question.
I want to use OTF fonts in XeLatex. To be concrete, here, for example, are some of the Linux Libertine fonts/fontfamilies available on my computer:
(Serif) /usr/share/texmf-dist/fonts/opentype/public/libertine/LinBiolinum_R.otf /usr/share/texmf-dist/fonts/opentype/public/libertine/LinBiolinum_RB.otf /usr/share/texmf-dist/fonts/opentype/public/libertine/LinBiolinum_RBO.otf /usr/share/texmf-dist/fonts/opentype/public/libertine/LinBiolinum_RI.otf ..... (Sans Serif) /usr/share/texmf-dist/fonts/opentype/public/libertine/LinLibertine_R.otf /usr/share/texmf-dist/fonts/opentype/public/libertine/LinLibertine_I.otf /usr/share/texmf-dist/fonts/opentype/public/libertine/LinLibertine_M.otf /usr/share/texmf-dist/fonts/opentype/public/libertine/LinLibertine_MB.otf ....
Suppose that I want to use to able to use all of LinLibertine fonts; roman, italics etc without defining \rm, \it individually. Is there a simple command(s) to do this? Can one do this for both LinLibertine (serif) and LinBiolinum (sans serif) at once. I was unable to find a suitable answer to what surely is a fundamental question.
What "name" does one use in such a command and how does one know what the "name" is. In the above example is it libertine or LinLibertine or an abbreviation (such as "cm") and does the "name" have capitals?