The standard LaTeX system format can be configured in several ways to suit the needs and resources of an installation. For example, the loading of fonts and font tables can be customised to match the font shapes, families and encodings normally used in text mode. Also, by producing the appropriate font definition files, the font tables themselves can be set up to take advantage of the available fonts and sizes. The loading of hyphenation patterns can be adjusted to cover the languages used; this has to be done as part of making the format since this is the only stage at which patterns can be loaded.
A complete list of these configuration possibilities can be found in
the distributed guide Configuration options for LaTeX2e
(cfgguide.tex). However, as it says there, the number of
configuration possibilities is strictly limited; we hope that having
read this far you will appreciate the reasons for this decision. One
consequence of this is that there is no provision for a general
purpose configuration file, or for adding extra code just before the
\dump of the format file.
This was a deliberate decision and we hope that everyone (yes, that includes you!) will support its intent. Otherwise there will be a rapid return to the very situation, of several incompatible versions of LaTeX 2.09, that originally prompted us to produce LaTeX2e: the new, and only, `Standard LaTeX'. This will make LaTeX unmaintainable and, hence, unmaintained (by us, at least).
Therefore you should not misuse the configuration files or other parts of the distribution to produce non-standard versions of LaTeX.
Some of the allowed configurations can result in a system that can produce documents that are no longer `formatting compatible'; for example, the use of different default fonts will most likely produce different line and page breaks. If you do produce a system that is configured in such a way that it is not `formatting compatible' then you should consider carefully the needs of users who need to create portable documents. A good way to provide for their needs is to make available, in addition, a standard form of LaTeX without any `formatting incompatible' customisations.