'PropertyValue' attribute usage

[#1]

How do you specify a property when using PropertyValue attribute ? 

for example, when we have 2 attributes, A and B

how do we specify that we want to use the PropertyValue of property A ? 

I tried

${PropertyValue:A}

but it did not work 

[#2]

Not sure I understand the question, which processor/property are you taking about? Basically if you want to access a flowfile attribute value in expression language you can use ${A} in case you have an attribute called A