[H-GEN] passwd and shadow files

[List Puppy]

For some distributions (eg: RHEL), using or not using a shadow file
for passwords, is an installation option.  As is the use of MD5 or DES
encoding for the passwords.

It appears that some distributions simply default to installing a
shadow file without giving the user a choice.

What I am curious about is; After the install is complete and a shadow
file has, or has not been implemented, how does the system know to use
the shadow file if it is there?

Is there a configuration file somewhere that tells the system, at boot
time, to use a shadow file instead of the password file?  If so;
where, and, what is its name?