Crochet supports 'options' that can be enabled from
the user configuration as

   option Example arg1 arg2

Options can modify the final image in a wide variety of ways: They can
alter a variety of internal configuration that affects how FreeBSD
work or kernel is built or installed.  They can add, remove, or modify
content in the final built image.

Generally speaking, options that affect the installed system take
effect after the basic system is installed and customized by the board
configuration.

Options can take arguments and it often makes sense to invoke
the same option multiple times with different arguments.
For example:
   option Package apache
   option Package mysql
   option Package php

(The 'option Package' has not yet been fully implemented; it's
in the works.)

If you're interested in adding a new option, look at
the existing options, especially option/Example/setup.sh
which has a lengthy description of how options work and
how to add one.