N+ in Pepi makes a depletion region which
can sweep out minority carriers that enter
its influence. Tied locally (sub! / vss!) it will
have a smaller depletion volume but return
substrate carriers to the substrate terminal.
Tied to vdd! it will present a larger "collection
target", hopefully a few charges more or less
won't bother the supply. But there are some
environments where much more substrate
carriers are generated.
The more "taps", the less deflection the well
or local substrate region will deflect from its
proper potential when presented with
displacement currents from parasitic
capacitances' node slewing. Voltage excursions
can enter the designed circuit by the "body
effect". A guardring is the most effective "tap"
geometry (fatter and closer is better).