I think what they mean is they are specially selecting certain wood for particular purposes.

Through the years his company has evolved from a simple sawmill operation to a lumber manufacturing facility with concentration yards, planer mills, several custom sorting mills, a custom dimension mill, and a pallet dimension operation. Musser
Lumber prides itself on its unique ability to offer quality lumber, customized to each customer's specifications at a very competitive price. Just ask our growing list of customers across North America, Europe and the Pacific Rim.

http://www.musserlumber.com/

Not all lumber can be used for any purpose, that is why lumber is graded. Grading considers hardness, grain structure, no. of knots etc. and determines what a particular piece of lumber is suited for.