Leveling flux is a mixture of more pure quality zinc chloride solution and ammonium Chloride with some special additives which make this flux more expensive than others.
It is in liquid form with ideal mixing ratio. 
This can be used to increase the Baume (Be) or Specific Gravity of the Flux Bath in between the Galvanizing process which increases the life of Galvanizing Pre Flux. 

Some special Features 

It is in liquid form so can be directly used to top up the Flux Bath in between the galvanizing process.
Increases the life of flux in flux bath.
Easy and safe to use as no prior mixing is required and can be directly poured in the flux bath which saves time and labour