@hsl I never heard about this term until now, but I can give an insight on what do I usually take in my backpack whenever I leave the house for a longer trip:
a compact umbrella
some tissues
a powerbank for my phone (despite nowadays my phone battery lasting more than a day and being replaceable)
a pen
a 0,75-1l bottle of water
All these usually fit in a small backpack like mine just right.