Use Value to read data directly from and write data directly to a string field component at runtime. Using the Value property is the same as using the AsString property. Use Value when you know the field component is a string field. Use AsString when working with a generic field component. 

When setting the Value property, TStringField can convert the assigned string from UTF-8 or ANSI to the character set used by the dataset, if the Transliterate property is true. Strings that have more characters than permitted by the value of the Size property are truncated when setting Value. 

AsString 

Size 

Transliterate