There are 4 ways a resource request from a specific part is handled. The ResourceFlowMode enum has more values, but there is no difference between some of them.
NO_FLOW
- Only the resources on the part the request originate from can be reached
 - The resource can only be used if 
flowStateistrue 
ALL_VESSEL / ALL_VESSEL_BALANCE
- The request will reach all parts on the vessel, excepted those for which the resource 
flowStateisfalse