Chelating agents in chemistry basically means a chemical that "binds." As with a lot of publicly sold chemicals, the ingredient "Chelating Agent" as it stands is completely ambiguous.
A chelate can be any number of things.
-Polyvinylpyrollidone (an ingredient in some tap water conditioners) bind to most polar molecules, making it a chelate.
-Another ingredient found on some water conditioners is EDTA (EthyleneDiamineTetraAcetic acid) binds to harmful metal ions that may be in tap water.
-Heavy metals such as copper (can be copper sulfate), cobalt, are also chelating agents for certain molecules.
So as you can see, a chemical ingredient label "chelating agent" could mean a whole number of things. As for it's use with ammonia, I have no clue.