@Pedro To actually answer your question, sometimes nothing, like in these bags i leave space for shopping:
https://lapolicegear.com/tactical-utility-shoulder-bag.html
The tail bag is out of production, cant show you an example, its a MOLLE bag too, like all these bags are,
as they all can be used on my tactical MOLLE vest that i have.
In the tailbag i keep a spare tube and some tools, and a siphon hose to feed those less endowed with a big ass gas tank.
The medical bags are self explanatory unless your name is Bruce.
To the front crash bars, my fire extingusher is in this MOLLE bag :
https://lapolicegear.com/condor-radio-pouch-multicam.html
because Smoky Bear is a friend of mine from way back.
I mean chit catches fire near me allot i dunno why.
These bags are meant for .30 cal belt ammo:
https://lapolicegear.com/tactical-ammo-pouch.html
Holds tie downs for the trailer for when my spare motor parts break and a bunch more space left over..
I love these, they do triple duty, from hydration to spare nuts, you name it.
https://lapolicegear.com/condor-h2o-pouch.html
All in all Molle bags were invented for motorcycles imho, heavy duty stiching and zippers, rip proof fabric, throw them on a vest or bike, and the prices arnt bad.
If i add weight i get to spread it around and keep it on the down low parts
If you wonder what Molle gear is see this:
https://en.wikipedia.org/wiki/MOLLE