The identifying value of the parameter.
Although always presented as a string, the data within Value must validate against the ProductParameterType of the parameter:
String = Any string.
Float = Any floating point number.
DateTime = Any date.
Multi = Any predefined value id from the predefined set of values for this parameter.
Single = Any predefined value id from the predefined set of values for this parameter.
Headline = Any string.
Tags = Any string, as part of a pipe-separated list. A string containing the pipe (|) character is not allowed.