The disadvantage of most implementations is the fixed size of the individual arrays in TEX. If new macro packages are published, like e.g. the new Font Selection Scheme of LaTEX, then you mostly reach one of the boundaries of the many arrays in TEX (in the above mentioned one, this is the String Pool) and the user gets the tidy message: ``you can ask a wizard to enlarge me''.
With the configuration file tex.cnf the user himself can now become a TEX-Wizard. In case the aforementioned message should appear, you simply change the size of the array that is too small.
Four types of information is contained in the configuration file: Searchpaths Preselections CodePage information Size of TEXarrays
The information is indicated as a keyword-value pair, where the keyword must begin at the beginning of the line and be separated from the value with spaces or tabs.
Keywords not recognized are ignored without a warning, you can use this to include comments in the file. Better is, however, the use of the percent sign `|after the percent sign is ignored.
The configuration file is read to the end, but you can stop reading before that by including a `|#|' as the first character in a line. This is meaningful when e.g. a longer comment follows after that.