Part-time 4x4 means that it cannot be used "all the time" on paved roads.
It's really meant for offroading only.  The only time it could be used on
the highway is if the highway were totally covered with some slick substance
like snow or ice.

Only full-time 4x4 systems can be used on the highway without regard to
highway conditions.  Full-time systems allow the front and rear tires to
rotate at different rpms as is needed when you're turning left or right.  A
part-time 4x4 system mechanically locks the front and rear axles together
which means they are forced to turn at the same rpms which is bad if you're
turning left or right... since the front tires must rotate faster in a turn
than the rear tires do.  A part-time 4x4 prevents that from happening, a
full-time system allows the front and rear tires to rotate at different rpms
from each other.

Don't shift into 4x4 with a part-time system unless you're offroad or the
roads are TOTALLY covered with snow, ice, mud, etc. which allows the front
and rear tires to slip on the surface as they need to as the front tires
rotate faster around curves or turns.

Jerry

Part time systems utilize two differentials. One on the front axle and
one on the rear.

Full time systems utilize a third differential (or a viscous coupling)
between the front and rear axles.

Nope that's it.... just the gear ratio unless it's a Select-Trac. Then
the low ratio is part time while the higher ratio can be either full
or part-time.

Why would you want part time if full time is available..... same
reason you might want a limited slip differential in a 2WD..... in
slippery conditions you can eliminate slip or spin in that third
centre differential by locking it up.

I have the Select-Trac and love it.