Lumber and many other materials are measured in lineal feet of lengths. You can get a lumber price quoted in lineal feet, but you may have to know the number of square feet it covers. For lumber and siding products, quite often, having the square foot amount instead of lineal feet makes more sense.

To convert an amount in lineal feet to square feet, you need to know the board's width.

And as a side note, I never have heard any use or mentioning of linear feet. It has always been lineal feet!