Thanks Borek comment.
1. Yes,I agree the demasking agents and cyanide complexs are the 'masking' category.
2. I think increase the selectivity is same as increase the EDTA react to the target metal. Therefore by controlling the pH, use of masking agents, solvent extraction ...are the methods or procedures for such purpose.
3.Some of anion such as phosphate in the sample will interfere the selectivity of EDTA. So this must remove before analysis.
4.Under some conditon, the metal does not effectively entre into the complexation reaction. For example determination of Cr and Fe in steel sample solution. Under cool condition,the EDTA react with Fe first and then heat the solution, the EDTA react with Cr.
5.Different indicator have different reaction rate.
Please correct me if I get wrong. Thanks.