First thing to remember about 5th is that they seem to largely assume you have already played previous editions and some bits just were not listed or not listed as clearly as they could have been because either it was missed or they just assumed you would know. Annoying at times, but it happened.
In the meantime, we refer you to page 100 in the core rules
QUOTE (Core pg 100)
Every character has a Condition Monitor that tells the player how much Physical and Stun damage they can take before falling unconscious.
So fill either one and you are down. Pretty straight forward.
This is also touched on in the survival skill:
QUOTE (Core pg 137)
If your Stun Condition Monitor overflows and you fall unconscious, you cannot make your Survival Tests, so you just keep taking
the damage (now Physical from overflow) until you’re either rescued or dead.
Again, implying that a full Stun bar means naptime.
Likewise the Resist Pain spell mentions:
QUOTE (Core pg 289)
Resist Pain allows the subject to ignore the pain of injuries, reducing penalties from Physical or Stun damage.
Each hit on the Spellcasting Test removes the effect of one box of damage from each of the subject’s Condition Monitor tracks.
It does not remove the damage itself; it only eliminates the modifiers.
If all of the boxes on the track are filled, the character still falls unconscious.
Again, telling us that a full Stun bar seems to mean go straight to bed, do not pass Go, do not collect 200 nuyen.
Disruption indirectly refers to this here:
QUOTE (Core pg 316)
Disrupted spells and rituals end, disrupted preparations lose their spell and become mundane,
disrupted mana barriers collapse, disrupted foci deactivate, disrupted spirits are banished, and disrupted living
creatures are knocked unconscious with a full Stun Condition Monitor.
This is sort of backward way of saying it
As does the Critter Power Engulf for Elemental water effects:
QUOTE (Core pg 396)
Victims who pass out from Stun damage continue to take
damage after falling unconscious, with the Stun damage
overflowing into Physical damage as normal.
Again implying that a full stun track leads to unconsciousness.
Interestingly the bioware Adrenaline Pump actually keeps you on your feet even when you should be unconscious due to Stun:
QUOTE (Core pg 459)
While the pump is active, you ignore injury modifiers
and don’t fall unconscious, even if your Stun Condition Monitor is filled.
The exception to the rule if you will...
Hope this helps,