The description of how LaTeX font attributes are turned into TeX
fonts is usually kept in a font definition file (.fd). The
file for family <family> in encoding <ENC> must be
called <enc><family>.fd: for example, ot1cmr.fd for Computer
Modern Roman with encoding OT1 or t1ptm.fd for Adobe Times with
encoding T1. Note that encoding names are converted to lowercase
when used as part of file names.
Whenever LaTeX encounters an encoding/family combination that it
does not know (e.g. if the document designer says
\fontfamily{ptm}\selectfont) then LaTeX attempts to load the
appropriate .fd file. ``Not known'' means: there was no
\DeclareFontFamily declaration issued for this encoding/family
combination. If the .fd file could not be found, a warning is
issued and font substitutions are made.
The declarations in the font definition file are responsible for telling LaTeX how to load fonts for that encoding/family combination.