Into beer there is a difference between apparent extract and real extract.
Apparent extract is the value, which the brewer is using in his daily life. Apparent degree of fermentation is calculated with the help of apparent extract, with this value the brewer assessed progress of fermentation and steers the processes.
Value of real extract is more important for the evaluation of finished product, because this value is used to calculate original wort. It is also possible to use this value to generate the real degree of fermentation. Parameter real extract corresponds to parameter total extract.