Code analogy

anything you want to change can be done in postMeta
only rarely you need to manually add stuff in the meta function
the policy is named policy because it enforces a standard on how to map database stuff to the application
in the meta function, this policy can be influenced
compare it to pregnancy
meta() is what happens inside the female
based on dna, policies, whatever food she happens to be eating etc, the baby is constructed
once it's born, you give it clothing etc.
sometimes however, you need to do surgery on the pre-born
if you wait until postmeta, it is either too late, or inefficient
which is why some stuff is done in meta()
e.g. if the womb is constructing a boy but you want a girl, you could let it be born first and then do a gender change
but that's a lot of overhead
so for performance reasons, you might change the gender before the utensils are appended to the unborn
(which is basically what things like $policy->setTypeAndParams do
(to make it a little more complete, even metanodes have a constructor, but that would then be comparable to conception, and you don't want to fiddle with stuff there yet)

also published on the ATK wiki .