Which type of devices are more sensitive to ESD ? What are the units to measure ESD
You do not "measure ESD" directly. You test
and retest a part subjected to increasing
"voltage" levels, as part of a standardized
threat-source model (human body, charged
device, machine model, back in the day, but
now supplemented by additional specs that
apply to particular application segments
(e.g. "dumb consumer on carpet" applied
to products that are attached to accessible
connectors like HDMI or USB chips).
The "number" is the last passing voltage,
where the part still meets both spec table
and any assigned parameter drift limits.
It is full pin-pin, for "handling" ESD (HBM,
CDM, MM) but special cases might be
applied to parts that need to meet powered,
in-system ESD / EOS threats (you would not
care about, say, the inward-facing enable
or data pins beyond sensible "handling ESD"
expectations, but demand more from the
connector-side pins against each other
and any exposed power / ground pins.
When you see a "500V HBM" part that means
it was OK after a one-time 500V "zap set"
(all pin-pin combos) and failed subsequently
(or, the testing was ceased after reaching an
"acceptable" (per the person doing it, or who
set the bar) outcome. Properly, you would
test-to-failure some population of parts and
report the highest all-passing level.