The data stage will be executed in 3 step:
- Pre initialization
In this step will be loaded all scripts with functions
that will be used later in the mods, so libraries.
Will be also executed a mod assertion script,
if you don't know what assertion are:
https://en.wikipedia.org/wiki/Assertion_(software_development)

- Contents initialization
In this step will be loaded entities, items, recipes...
only what will be loaded in first data stage.
All modification to vanilla game or other related things
will be executed later in the data updates stage

All parts must be loaded with the appropriate loader file
that is sort of "...prototypes/entities-initialization"
this is for keep main data scripts slim,
see in that files for more info.

The optional sontents initialization is the step where
is loaded all mod optional part (chosen in mod option)

- Compatibility initialization (data stage)
In this step will be executed some scripts
for make compatibility operations with other mods,
in particular the compatibility operations that
can or must be executed in data stage.