"Mill Select" means the mill has selected the wood to meet the "Tight Knot" grade rule in the books.
There is no limit to the number of knots but there is a limit to the size and the type of knot.
Other grade names that equate to our definition of "Mill Select" are STK (Select Tight Knot) Select Knotty, Architectural Knotty, #2 and better.
This is a great wall to show "Mill Select". Note the sound tight knots and the size is smaller than is allowable in "Customer Select".
I also want to draw attention to the number of knots. "Mill Select" is a knotty look, some boards will have more and some fewer just know there is no limit to the number of knots.
This was 1x8 Bevel stained Olympic "Cedar" Semi-transparent, Oil Based on the rough face. The official mill grade description was Select Tight Knot or STK - Buffalo Lumber "Mill Select" grade equivalent.
For "Select Knotty" the knots must be tight and sound varying in size from approximately 2" in 1x6" and increasing by 1/2" in size up to 3 1/2" in 12". These are "nominal" widths - 6", 8", 10", 12". A 1x6" nominal board which will have an actual width of 5" when milled to a Channel, dutch, Tongue & Groove or Bevel pattern.
In Buffalo Layman terms this means that 5" board will have knots up to approximately 2 inches in diameter through out the length.
When we zoom in you can see the knot structure a little better.
Mill Select is graded to meet the "Tight Knot" description in any given rule book and is the first official "Appearance Grade" tier for commercial projects where codes have to be met.
These grade rules represent the maximum to expect per single board - NOT all boards. Some percentage of boards will have fewer or smaller knots per 12' length.